hydrodynamic modeling of short channel devices using an upwind flux vector splitting scheme upwind flux vector splitting is presented for hydrodynamic modeling of 1d and 2d deep submicron semiconductor devices this approach splits the jacobian matrices of the hydrodynamic system into two parts corresponding to forward and backward running fluxes and an upwind difference is applied to each split flux influences of the boundary discontinuity of electric field and velocity on transport results in the 2d deep submicron devices are discussed the study demonstrates that the developed method is capable of handling large discontinuity of the boundary conditions and highly non linear source terms in short channel semiconductor devices

advances in residual vector quantization a review advances in residual vector quantization rvq are surveyed definitions of joint encoder optimality and joint decoder optimality are discussed design techniques for rvqs with large numbers of stages and generally different encoder and decoder codebooks are elaborated and extended fixed rate rvqs and variable rate rvqs that employ entropy coding are examined predictive and finite state rvqs designed and integrated into neural network based source coding structures are revisited successive approximation rvqs that achieve embedded and refinable coding are reviewed a new type of successive approximation rvq that varies the instantaneous block rate by using different numbers of stages on different blocks is introduced and applied to image waveforms and a scalar version of the new residual quantizer is applied to image subbands in an embedded wavelet transform coding system

comparison between dpcm and hadamard transform coding in the composite coding of the ntsc color tv signal since differential pulse code modulation dpcm and orthogonal transform coding otc are the most fundamental methods of high efficiency coding bit reduction method it is important to clarify the basic coding characteristics of these methods and the difference between them in order to utilize the bit reduction method effectively this paper theoretically as well as experimentally compares the coding efficiency of a dpcm having a two dimensional predictor with that of a two dimensional hadamard transform coding method htc in the intrafield coding of the ntsc composite signal the comparison evidenced that the distinctive difference in coding characteristics between dpcm and htc depends greatly on the power level of carrier chrominance signals that is it is confirmed theoretically and experimentally that the coding efficiency of the htc is far lower than that of the dpcm in the case of a signal having a high power level carrier chrominance signal such as a color bar signal

treewidth computations ii lower bounds for several applications it is important to be able to compute the treewidth of a given graph and to find tree decompositions of small width reasonably fast good lower bounds on the treewidth of a graph can amongst others help to speed up branch and bound algorithms that compute the treewidth of a graph exactly a high lower bound for a specific graph instance can tell that a dynamic programming approach for solving a problem is infeasible for this instance this paper gives an overview of several recent methods that give lower bounds on the treewidth of graphs

two fast nearest neighbor searching algorithms for image vector quantization in this paper two efficient codebook searching algorithms for vector quantization vq are presented the first fast search algorithm utilizes the compactness property of signal energy on transform domain and the geometrical relations between the input vector and every codevector to eliminate those codevectors that have no chance to be the closest codeword of the input vector it achieves a full search equivalent performance as compared with other fast methods of the same kind this algorithm requires the fewest multiplications and the least total times of distortion measurements then a suboptimal searching method which sacrifices the reconstructed signal quality to speed up the search of nearest neighbor is presented this algorithm performs the search process on predefined small subcodebooks instead of the whole codebook for the closest codevector experimental results show that this method not only needs less cpu time to encode an image but also encounters less loss of reconstructed signal quality than tree structured vq does

encoding and progressive transmission of still pictures in ntsc composite format using transform domain methods coding schemes suitable for progressive transmission of still pictures in ntsc composite format are studied transform coding methods based on the discrete cosine transform dct and the walshhadamard transform wht are used the transform coefficients are segmented into groups having similar properties variable blocklength to variable length codes are used to encode the quantized coefficients for each of the groups progressive transmission is achieved by sending the coefficients belonging to particular groups in successive passes over the image results are presented for coding the ntsc signal sampled at four times the color subcarrier frequency with an orthogonal structure and at twice the subcarrier frequency with a hexagonal structure

a comparative study of characteristic based algorithms for the maxwell equations characteristic based finite difference and finite volume schemes have been developed for solving the three dimensional maxwell equations in the time domain a detailed eigenvector analysis for the maxwell equations in a general curvilinear coordinate has also been completed to provide a basic framework for future finite difference schemes although the basic concepts of the two algorithms are identical the detailed formulations are vastly different for achieving split flux vectors according to the sign of the eigenvalues a comparative study of these algorithms applied to an oscillating electric dipole is carried out to assess their relative merit for further development in spherical coordinates second order windward numerical simulations of the radiating phenomenon are closely comparable in terms of accuracy and efficiency these methods also demonstrate an ability to suppress reflected waves from the truncated boundary by a simple compatibility condition

reversible intraframe compression of medical images the performance of several reversible intraframe compression methods is compared by applying them to angiographic and magnetic resonance mr images reversible data compression involves two consecutive steps decorrelation and coding the result of the decorrelation step is presented in terms of entropy because huffman coding generally approximates these entropy measures within a few percent coding has not been investigated separately it appears that a hierarchical decorrelation method based on interpolation hint outperforms all other methods considered the compression ratio is around 3 for angiographic images of 8 9 b pixel but is considerably less for mr images whose noise level is substantially higher

low entropy image pyramids for efficient lossless coding an efficient image source coding technique gives good compression performance at low computational complexity this research introduces an efficient coding technique based on pyramid coding that involves transforming an image into an equivalent lower entropy form prior to lossless coding the proposed method is also a multiresolution technique that facilitates progressive image transmission

hybrid picture coding with wavelet transform and overlapped motion compensated interframe prediction coding a novel video coding scheme using an orthonormal wavelet transform is proposed the wavelet transform is used in a motion compensated interframe coder in which a blockless motion compensation technique is employed to increase efficiency of wavelet transform coding a new scanning method for wavelet coefficients is also proposed which is rather different from subband coding simulation work is carried out to evaluate the proposed coding method significant improvement in subjective quality is obtained over that obtained with conventional hybrid coding methods that use blockwise motion compensation and dct some improvement has also been realized in the signal to noise ratio although wavelet coding is still in its early stages of development it appears to hold great promise for motion picture coding

upwind relaxation algorithms for the navier stokes equations analyse des erreurs de troncature de la differenciation spatiale et analyse de la stabilite de l algorithme de relaxation calcul pour une serie d ecoulements a l aide de la forme couche mince des equations calcul du decollement de couche limite induit par chocs

the lot transform coding without blocking effects an exact derivation of an optimal lapped orthogonal transform lot is presented the optimal lot is related to the discrete cosine transform dct in such a way that a fast algorithm for a nearly optimal lot is derived compared to the dct the fast lot requires about 20 30 more computations mostly additions an image coding example demonstrates the effectiveness of the lot in reducing blocking effects the lot actually leads to slightly smaller signal reconstruction errors than does the dct

reversible decorrelation method for progressive transmission of 3 d medical image in this paper the authors present a new reversible decorrelation method of three dimensional 3 d medical images for progressive transmission progressive transmission of an image permits gradual improvement of image quality while being displayed when the amount of image data is very large as a 3 d medical image the progressive transmission plays an important role in viewing or browsing the image the data structure presented in this paper takes account of interframe correlation as well as intraframe correlation of the 3 d image this type of data structure has been termed the 3 d hierarchy embedded differential image 3 d hedi as was derived from the earlier hedi structure kim et al 1995 experiments were conducted to verify the performance of 3 d hedi in terms of the decorrelation efficiency as well as the progressive transmission efficiency it is compared with those of conventional hierarchy interpolation hint two dimensional 2 d hedi and differential pulse code modulation dpcm experimental results indicate that 3 d hedi outperforms hint 2 d hedi and dpcm in both decorrelation efficiency as well as the progressive transmission efficiency on 3 d medical images

on the symmetric form of systems of conservation laws with entropy abstract this paper reviews the symmetrizability of systems of conservation laws which possess entropy functions symmetric formulations in conservation form for the equations of gas dynamics are presented

fundamental domains for congruence subgroups of sl2 in positive characteristic abstract in this work we construct fundamental domains for congruence subgroups of sl 2 f q t and pgl 2 f q t our method uses gekeler s description of the fundamental domains on the bruhat tits tree x x q 1 in terms of cosets of subgroups we compute the fundamental domains for a number of congruence subgroups explicitly as graphs of groups using the computer algebra system magma

btc vq dct hybrid coding of digital images a hybrid btc vq dct block truncation coding vector quantization and discrete cosine transform image coding algorithm is presented the algorithm combines the simple computation and edge preservation properties of btc and the high fidelity and high compression ratio of adaptive dct with the high compression ratio and good subjective performance of vq and can be implemented with significantly lower coding delays than either vq or dct alone the bit map generated by btc is decomposed into a set of vectors which are vector quantized since the space of the btc bit map is much smaller than that of the original 8 b image a lookup table based vq encoder has been designed to fast encode the bit map adaptive dct coding using residual error feedback is implemented to encode the high mean and low mean subimages the overall computational complexity of btc vq dct coding is much less than either dct and vq while the fidelity performance is competitive the algorithm has strong edge preserving ability because of the implementation of btc as a precompress decimation the total compression ratio is about 10 1

adaptive dct coding of video signals a coefficient by coefficient adaptive discrete cosine transform dct is investigated the interframe dct has been known to possess an adverse effect in that it generates mosquito and blocking noise by analyzing dct for the intraframe and the interframe video signals a new adaptive intra interframe dct is developed cases without and with the movement compensation are discussed separately through simulations it is shown that a substantial improvement gain 1 0 5 0 db sub p p or 0 2 2 0 db sub p p respectively can be obtained by the new adaptive method the improvement is brought about by the reduction of the blocking noise conventionally generated by the coarse quantization of the higher order interframe dct coefficients by calculating the improvement bound a further possibility is discussed

distributions of the two dimensional dct coefficients for images for a two dimensional discrete cosine transform dct image coding system there have been different assumptions concerning the distributions of the transform coefficients this paper presents results of distribution tests that indicate that for many images the statistics of the coefficients are best approximated by a gaussian distribution for the dc coefficient and a laplacian distribution for the other coefficients furthermore from a simulation of the dct coding system it is shown that the assumption that the coefficients are laplacian yields a higher actual output signal to noise ratio and a much better agreement between theory and simulation than the gaussian assumption

abelian subvarieties of drinfeld jacobians and congruences modulo the characteristic we relate the existence of frobenius morphisms into the jacobians of drinfeld modular curves to the existence of congruences between cusp forms

multiresolution multiresource progressive image transmission this paper presents a new progressive image transmission pit design algorithm in which the resolution and resources rate or distortion and storage size at each transmission stage are allowed to be prespecified this algorithm uses the wavelet transform and tree structured vector quantizer tsvq techniques the wavelet transform is used to obtain a pyramid structure representation of an image the vector quantizer technique is used to design a tsvq for each subimage so that all the subimages that constitute the image at the current stage can be successively refined according to the resources available at that stage the resources assigned to each subimage for the successive refinement is determined to optimize the performance at the current stage under the resource constraints normally the resource constraints at each stage are determined by the specification of the transmission time or distortion for image data and the storage complexity of the tsvq the resolution at each stage is determined specified according to the application or as part of the design process to optimize the visual effect

prioritized dct for compression and progressive transmission of images an approach is based on the block discrete cosine transform dct the novelty of this approach is that the transform coefficients of all image blocks are coded and transmitted in absolute magnitude order the resulting ordered by magnitude transmission is accomplished without sacrificing coding efficiency by using partition priority coding coding and transmission are adaptive to the characteristics of each individual image and therefore very efficient another advantage of this approach is its high progression effectiveness since the largest transform coefficients that capture the most important characteristics of images are coded and transmitted first this method is well suited for progressive image transmission further compression of the image data is achieved by multiple distribution entropy coding a technique based on arithmetic coding experiments show that the approach compares favorably with previously reported dct and subband image codecs

image compression with variable block size segmentation high quality variable rate image compression is achieved by segmenting an image into regions of different sizes classifying each region into one of several perceptually distinct categories and using a distinct coding procedure for each category segmentation is performed with a quadtree data structure by isolating the perceptually more important areas of the image into small regions and separately identifying larger random texture blocks since the important regions have been isolated the remaining parts of the image can be coded at a lower rate than would be otherwise possible high quality coding results are achieved at rates between 0 35 and 0 7 b p depending on the nature of the original image and satisfactory results have been obtained at 0 25 b p

zonal sampling and bit allocation ht coefficients in image data compression in this paper we report our investigation on zonal sampling probability fitting tests and bit allocation for hadamard transform image data compression the energy distribution of the transform coefficients suggests a sampling strategy which retains as much energy as possible for a fixed number of coefficients the probability fitting tests indicate that laplacian is a better model for non dc coefficients while rayleigh or gaussian fits better for the dc coefficient finally the bit assignment is formulated as a constrained optimization problem the mean square error performance is derived as a function of the number of retained coefficients for various average bit rates the results are useful for designing a zonal coding system with optimum retained zone size and shape as well as their corresponding bit assignment

analogue of the degree conjecture over function fields under a certain assumption similar to manin s conjecture we prove an upper bound on the degree of modular parametrizations of elliptic curves by drinfeld modular curves which is the function field analogue of the conjectured bound over the rational numbers

recognizing hyperelliptic graphs in polynomial time abstract based on analogies between algebraic curves and graphs baker and norine introduced divisorial gonality a graph parameter for multigraphs related to treewidth multigraph algorithms and number theory various equivalent definitions of the gonality of an algebraic curve translate to different notions of gonality for graphs called stable gonality and stable divisorial gonality we consider so called hyperelliptic graphs multigraphs of gonality 2 in any meaning of graph gonality and provide a safe and complete set of reduction rules for such multigraphs this results in an algorithm to recognize hyperelliptic graphs in time o m n log n where n is the number of vertices and m the number of edges of the multigraph a corollary is that we can decide with the same runtime whether a two edge connected graph g admits an involution such that the quotient g is a tree

combined techniques of singular value decomposition and vector quantization for image coding the combination of singular value decomposition svd and vector quantization vq is proposed as a compression technique to achieve low bit rate and high quality image coding given a codebook consisting of singular vectors two algorithms which find the best fit candidates without involving the complicated svd computation are described simulation results show that the proposed methods are better than the discrete cosine transform dct in terms of energy compaction data rate image quality and decoding complexity

image compression using adaptive vector quantization the paper demonstrates the application of adaptive vector quantization techniques to the coding of monochromatic and color pictures codebooks of representative vectors are generated for different portions of the image by means of extensive simulations the performance of the coder for monochrome images is estimated as a function of the various coding parameters vector dimensionality number of representative vectors and degree of adaptivity it is shown that for a vector size of dimension 4 there is a zone of operation ranging from 1 0 to 1 5 bits pixel where adaptive vector quantization is advantageous despite the simplicity of the decoder the performance of the adaptive vector quantizer is found to be comparable with block transform coding schemes for both monochrome and color pictures

scene adaptive coder an efficient single pass adaptive bandwidth compression technique using the discrete cosine transform is described the coding process involves a simple thresholding and normalization operation on the transform coefficients adaptivity is achieved by using a rate buffer for channel rate equalization the buffer status and input rate are monitored to generate a feedback normalization factor excellent results are demonstrated for coding of color images at 0 4 bits pixel corresponding to real time color television transmission over a 1 5 mbit s channel

a new finite element formulation for computational fluid dynamics viii the galerkin least squares method for advective diffusive equations abstract galerkin least squares finite element methods are presented for advective diffusive equations galerkin least squares represents a conceptual simplification of supg and is in fact applicable to a wide variety of other problem types a convergence analysis and error estimates are presented

image sequence coding using vector quantization in this paper a new interframe coding technique based upon vector quantization is presented this algorithm has as its basis twodimensional block vector quantization at the frame level onto which is grafted the concept of adaptive codebook replenishment and frame replenishment this algorithm includes three strategies label replenishment label replenishment with mean shift and label replenishment with mean shift and selective codeword replacement the last two strategies efficiently update the codebook to track the changes in local statistics on a frame basis compared with three dimensional block vector quantization 16 for bit rates between 0 5 and 0 65 the normalized mean squared error nmse is reduced by a factor ranging between 2 and 2 5 by the use of replenishment

numerical problems in semiconductor simulation using the hydrodynamic model a second order finite difference scheme in this paper a second order total variation diminishing tvd finite difference scheme of upwind type is employed for the numerical approximation of the classical hydrodynamic model for semiconductors proposed by blotekjaer and baccarani wordeman in particular the high order hyperbolic fluxes are evaluated by a suitable extrapolation on adjacent cells of the first order fluxes of roe while total variation diminishing is achieved by limiting the slopes of the discrete riemann invariants using the so called flux corrected transport approach extensive numerical simulations are performed on a submicron n n n ballistic diode the numerical experiments show that the spurious oscillations arising in the electron current are not completely suppressed by the tvd scheme and can lead to serious numerical instabilities when the solution of the hydrodynamic model is non smooth and the computational mesh is coarse the accuracy of the numerical method is investigated in terms of conservation of the steady electron current the obtained results show that the second order scheme does not behave much better than a corresponding first order one due to a poor performance of the slope limiters caused by the presence of local extrema of the riemann invariant associated with the hyperbolic system

numerical resolution for a compressible two phase flow model based on the theory of thermodynamically compatible systems this paper is concerned with the numerical solution of the equations governing two phase gas solid mixture in the framework of thermodynamically compatible systems theory the equations constitute a non homogeneous system of nonlinear hyperbolic conservation laws a total variation diminishing tvd slope limiter centre slic numerical scheme based on the splitting approach is presented and applied for the solution of the initial boundary value problem for the equations the model equations and the numerical methods are systematically assessed through a series of numerical test cases strong numerical evidence shows that the model and the methods are accurate robust and conservative the model correctly describes the formations of shocks and rarefactions in two phase gas solid flow

adaptive cosine transform image coding with constant block distortion an adaptive block discrete cosine transform dct coding scheme is implemented with the same average distortion designated for each block this constant distortion designation not only has perceptual advantages but also allows the rate to vary adjusting to the changing spectral characteristics among the blocks the successful execution of this scheme requires a different spectral estimate for each block to keep overhead and computation within limits a novel technique is introduced by which a two dimensional block spectrum is characterized by a one dimensional autoregressive model simulations with images of natural scenes and medical radiology provide reconstructions with nearly uniform block distortion and very high visual and measurable quality at low rates

progressive image transmission using vector quantization on images in pyramid form a progressive image transmission scheme in which vector quantization is applied to images represented by pyramids is proposed a mean pyramid representation of an image is first built up by forming a sequence of reduced size images by averaging over blocks of 2 2 pixels a difference pyramid is then built up by taking the differences between successive levels in the mean pyramid progressive transmission is achieved by sending all the nodes in the difference pyramid starting from the top level and ending at the bottom level the kth approximate image can be formed by adding the information of level k to the previously reproduced k 1 st approximation to gain efficiency vector quantization is applied to the difference pyramid of the image on a level by level basis if the errors due to quantization at level k are properly delivered and included in the next level k 1 then it is demonstrated that the original image can be reconstructed an entropy coder is used to encode the final residual error image losslessly thus ensuring perfect reproduction of the original image the experiments demonstrate that it is possible to achieve simultaneously lossless and progressive transmission with compression at the intermediate level the use of vector quantization results in a coding gain over that obtained using only a huffman coder excellent reproduction is achieved at a bit rate of only 0 06 bits pixel

a new conformal invariant and its applications to the willmore conjecture and the first eigenvalue of compact surfaces let m be a compact riemannian manifold with a fixed conformal structure then we introduce the concept of conformal volume of m in the following manner for each branched conformal immersion q9 of m into the unit sphere s n we consider the set of all branched conformal immersions obtained by composi t ion of qo with the conformal automorphisms of s we let vc n qg be the max imum volume of these branched immersions the conformal volume of m is defined to be the infimum of v n q0 where qo ranges over all branched conformal immersions of m into the unit sphere s in this paper we study the case when m is a compact surface and we call the conformal volume of m to be the conformal area of m we demonst ra te that this conformal invariant is non trivial in fact we prove that if there exists a minimal immersion of m into s where coordinate functions are first eigenfunctions then the conformal area of m is given by the area of m with respect to the induced metric this enables us to compute the conformal area for several surfaces for example the conformal area of r p z is 6n and the conformal area of the square torus is 27c 2 we believe that the computa t ion of the conformal area for general surfaces will be very impor tant in studying the geometry of compact surfaces we demonstrate this claim by applying the concept of conformal area to two different branches of surface theory the first application is to study the total curvature of a compact surface in r this problem has a long history fenchel and fary 9 proved that for a closed curve o in r slk 27c where k is its curvature then milnor 12

a novel encoding algorithm for vector quantization using transformed codebook in this paper a novel encoding algorithm for vector quantization is presented our method uses a set of transformed codewords and partial distortion rejection to determine the reproduction vector of an input vector experimental results show that our algorithm is superior to other methods in terms of the computing time and number of distance calculations compared with available approaches our method can reduce the computing time and number of distance calculations significantly compared with the available best method of reducing the number of distance computations our approach can reduce the number of distance calculations by 32 3 67 1 compared with the best encoding algorithm for vector quantization our method can also further reduce the computing time by 19 7 23 9 the performance of our method is better when a larger codebook is used and is weakly correlated to codebook size

progressive refinement using local variance estimators when transmitting an image over a slow communications link it is desired to provide the best display possible in early stages of the transmission progressive refinement attempts to continuously minimize the deviation between the image and its displayed approximation it is shown that this minimization is accomplished by transmitting average values of regions of the image at increasingly fine levels of resolution the optimal refinement strategy is to subdivide the region which maximizes the product of area and pixel variance typically the receiving process cannot determine the pixel variance estimators of the variance can be computed from information already received are developed they are applied to refinement over the spatial distribution of values and simultaneously to refinement over their gray scale quantization the estimators are shown to apply equally well to monochrome and full color image these techniques are implemented with no transmission overhead and are shown to generate qualitatively and quantitatively superior displays

numerical solution of two carrier hydrodynamic semiconductor device equations employing a stabilized finite element method abstract a space time galerkin least squares finite element method was presented in 1 for numerical simulation of single carrier hydrodynamic semiconductor device equations the single carrier hydrodynamic device equations were shown to resemble the ideal gas equations and galerkin least squares finite element method originally developed for computational fluid dynamics equations 16 was extended to solve semiconductor device applications in this paper the space time galerkin least squares finite element method is further extended and generalized to solve two carrier hydrodynamic device equations the proposed formulation is based on a time discontinuous galerkin method in which physical entropy variables are employed a standard galerkin finite element method is applied to the poisson equation numerical simulations are performed on the coupled poisson and the two carrier hydrodynamic equations employing a staggered approach a mathematical analysis of the time dependent multi dimensional hydrodynamic model is performed to determine well posed boundary conditions for electrical contacts the number of boundary conditions that need to be specified for the hydrodynamic equations at inflow and outflow boundaries of the device are derived example boundary conditions that are based either on physical and or mathematical basis are presented stability of the numerical algorithms is addressed the space time galerkin least squares finite element method and the standard galerkin finite element method for the hydrodynamic and poisson equations respectively are shown to be stable specifically a clausius duhem inequality a basic stability requirement is derived for the hydrodynamic equations and the proposed numerical method automatically satisfies this stability requirement numerical simulations are performed on one and two dimensional two carrier p n diodes and the results demonstrate the effectiveness of the proposed numerical method

progressive image transmission by transform coefficient residual error quantization a progressive image transmission scheme based on iterative transform coding structure is proposed for application in interactive image communication over low bandwidth channels the scheme not only provides progressive transmission but also guarantees lossless reproduction combined with a degree of compression the image to be transmitted undergoes an orthogonal transform and the transform coefficients are quantized scalar or vector before transmission the novelty is that the residual error array due to quantization is iteratively fedback and requantized scalar or vector the coded residual error information is progressively transmitted and utilized in reconstructing the successive approximations it is shown that the average reconstruction error variance converges to zero as the number of iterative stages approaches infinity in practice lossless reproduction can be achieved with a small number of iterations by using an entropy coder on the final residual error image computer simulation results demonstrate the effectiveness of the technique

a combinatorial li yau inequality and rational points on curves we present a method to control gonality of nonarchimedean curves based on graph theory let k denote a complete nonarchimedean valued field we first prove a lower bound for the gonality of a curve over the algebraic closure of k in terms of the minimal degree of a class of graph maps namely one should minimize over all so called finite harmonic graph morphisms to trees that originate from any refinement of the dual graph of the stable model of the curve next comes our main result we prove a lower bound for the degree of such a graph morphism in terms of the first eigenvalue of the laplacian and some volume of the original graph this can be seen as a substitute for graphs of the li yau inequality from differential geometry although we also prove that the strict analogue of the original inequality fails for general graphs finally we apply the results to give a lower bound for the gonality of arbitrary drinfeld modular curves over finite fields and for general congruence subgroups vargamma of vargamma 1 that is linear in the index vargamma 1 vargamma with a constant that only depends on the residue field degree and the degree of the chosen infinite place this is a function field analogue of a theorem of abramovich for classical modular curves we present applications to uniform boundedness of torsion of rank two drinfeld modules that improve upon existing results and to lower bounds on the modular degree of certain elliptic curves over function fields that solve a problem of papikian

comparative performance of pyramid data structures for progressive image transmission a review of various pyramid data structures for progressive image transmission focusing on the steps involved in forming the pyramid and the performance comparisons is presented the simple mean pyramid its variants including the truncated mean reduced sum difference reduced difference and s transform pyramids and the more general laplacian pyramid are discussed it is also shown that the use of an interpolation function improves the quality of the full resolution approximations both objectively and subjectively

on the uniform boundedness conjecture for drinfeld modules in analogy with the famous theorems of mazur and merel on the torsion subgroups of elliptic curves one can formulate similar conjectures for the torsion points of drinfeld modules we prove some partial results for rank 2 drinfeld q t modules for example the uniform boundedness of the primary torsion

reversible 3 d decorrelation of medical images two methods namely differential pulse code modulation dpcm and hierarchical interpolation hint are considered it is shown that hint cannot be extended straightforwardly to 3 d images as contrasted with dpcm a 3 d hint is therefore proposed which is based on a combination of 2 d and 3 d filters both decorrelation methods were applied to three dimensional computed tomography ct magnetic resonance mr and single photon emission ct spect images it was found that a 3 d approach is optimal for some studies while for other studies 2 d or even 1 d decorrelation performs better the optimal dimensionality of dpcm is related to the magnitudes of the local correlation coefficients ccs however the nonlocal nature of hint makes the local correlation coefficients useless as indicators of the dimensionality a better candidate is the image voxel size for images with cubic or nearly cubic voxels 3 d hint is generally optimal for images in which the slice thickness is large compared to the pixel size a 2 d intraslice hint is best in general the increase in efficiency obtained by extending 2 d decorrelation method to 3 d is small

modified jpeg huffman coding it is a well observed characteristic that when a dct block is traversed in the zigzag order the ac coefficients generally decrease in size and the run length of zero coefficients increase in number this article presents a minor modification to the huffman coding of the jpeg baseline compression algorithm to exploit this redundancy for this purpose dct blocks are divided into bands so that each band can be coded using a separate code table three implementations are presented which all move the end of block marker up in the middle of dct block and use it to indicate the band boundaries experimental results are presented to compare reduction in the code size obtained by our methods with the jpeg sequential mode huffman coding and arithmetic coding methods the average code reduction to the total image code size of one of our methods is 4 our methods can also be used for progressive image transmission and hence experimental results are also given to compare them with two three and four band implementations of the jpeg spectral selection method

a new finite element formulation for computational fluid dynamics x the compressible euler and navier stokes equations a space time element method is presented for solving the compressible euler and navier stokes equations the proposed formulation includes the variational equation predictor multi corrector algorithms and boundary conditions the variational equation is based on the time discontinuous galerkin method in which the physical entropy variables are employed a least squares operator and a discontinuity capturing operator are added resulting in a high order accurate and unconditionally stable method implicit explicit predictor multi corrector algorithms applicable to steady as well as unsteady problems are presented techniques are developed to enhance their efficiency implementation of boundary conditions is addressed in particular a technique is introduced to satisfy nonlinear essential boundary conditions and a consistent method is presented to calculate boundary fluxes numerical results are presented to demonstrate the performance of the method

a visually weighted quantization scheme for image bandwidth compression at low data rates source encoding of images for bandwidth compression has become attractive in recent years because of decreasing hardware costs by combining the source encoding approach with transform coding techniques it is possible to obtain good image quality at low data rates the general aspects of such a system are presented the design of the quantizer for transform coefficients which is the major source of error associated with the compression process is considered using a visual fidelity criterion and subject to the constraint that the entropy of the quantizer be a prespecified quantity a visually weighted suboptimal quantization scheme is developed to take into account the relative importance of different transform coefficients to the human visual system

a perceptually motivated three component image model part ii applications to image compression for past i see ibid vol 4 no 4 p 405 1995 the use of the image model of part i is investigated in the context of image compression the model decomposes the image into a primary component that contains the strong edge information a smooth component that represents the background slow intensity variations and a texture component that contains the textures the primary component which is known to be perceptually important is encoded separately by encoding the intensity and geometric information of the strong edge brim contours two alternatives for coding the smooth and texture components are studied entropy coded adaptive dct and entropy coded subband coding it is shown via simulations that the proposed schemes which can be thought of as a hybrid of waveform coding and feature based coding techniques result in both subjective and objective performance improvements over several other image coding schemes and in particular over the jpeg continuous tone image compression standard these improvements are especially noticeable at low bit rates furthermore it is shown that a perceptual tuning based on the contrast sensitivity of the human visual system can be used in the dct based scheme which in conjunction with the three component model leads to additional subjective performance improvements

moment closure hierarchies for kinetic theories this paper presents a systematicnonperturbative derivation of a hierarchy of closed systems of moment equations corresponding to any classical kinetic theory the first member of the hierarchy is the euler system which is based on maxwellian velocity distributions while the second member is based on nonisotropic gaussian velocity distributions the closure proceeds in two steps the first ensures that every member of the hierarchy is hyperbolic has an entropy and formally recovers the euler limit the second involves modifying the collisional terms so that members of the hierarchy beyound the second also recover the correct navier stokes behavior this is achieved through the introduction of a generalization of the bgk collision operator the simplest such system in three spatial dimensions is a 14 moment closure which also recovers the behavior of the grad 13 moment system when the velocity distributions lie near local maxwellians the closure procedure can be applied to a general class of kinetic theories

the laplacian pyramid as a compact image code we describe a technique for image encoding in which local operators of many scales but identical shape serve as the basis functions the representation differs from established techniques in that the code elements are localized in spatial frequency as well as in space pixel to pixel correlations are first removed by subtracting a lowpass filtered copy of the image from the image itself the result is a net data compression since the difference or error image has low variance and entropy and the low pass filtered image may represented at reduced sample density further data compression is achieved by quantizing the difference image these steps are then repeated to compress the low pass image iteration of the process at appropriately expanded scales generates a pyramid data structure the encoding process is equivalent to sampling the image with laplacian operators of many scales thus the code tends to enhance salient image features a further advantage of the present code is that it is well suited for many image analysis tasks as well as for image compression fast algorithms are described for coding and decoding

assessment of a high resolution centered scheme for the solution of hydrodynamical semiconductor equations hydrodynamical models are suitable to describe carrier transport in submicron semiconductor devices these models have the form of nonlinear systems of hyperbolic conservation laws with source terms coupled with poisson s equation in this article we examine the suitability of a high resolution centered numerical scheme for the solution of the hyperbolic part of these extended models in one space dimension because of the lack of physically significant exact analytical solutions the method is assessed against a benchmark for the system of compressible unsteady euler equations with source terms which has an exact solution the latter is shown to be nearly identical to the numerical one the method is then used to solve the extended hydrodynamical model em based on the maximum entropy closure recently introduced by anile romano and russo simulating a ballistic diode n n n which models a metal oxide semiconductor field effect transistor mosfet channel results are presented for the reduced and full equation em formulation at steady state for an initially discontinuous electron density at the junctions transient results show the evolution of highly nonlinear waves emanating from the neighborhood of the junctions

a finite element formulation for the hydrodynamic semiconductor device equations abstract a new formulation employing the galerkin least squares finite element method is presented for the simulation of the hydrodynamic model of semiconductor devices numerical simulations are performed on the coupled poisson and hydrodynamic equations for one carrier devices the hydrodynamic equations for a single carrier i e for the electrons or holes resemble the compressible navier stokes equations with the addition of highly nonlinear source terms and without the viscous terms the governing equations are nondimensionalized to improve the conditioning on the resulting system of equations and the efficiency of the numerical algorithms furthermore to establish the stability of the discrete solution the system of hydrodynamic equations is symmetrized by considering generalized entropy functions a staggered solution strategy is employed to treat the coupled hydrodynamic and poisson equations numerical results are presented for one dimensional and two dimensional one carrier n n n devices the presence of velocity overshoot has been observed and it is recognized that the heat flux term plays an important role in the simulation of semiconductor devices employing the hydrodynamic model

theoretical comparison between dpcm and transform coding regarding the robustness of coding performance for variation of picture statistics since dpcm and transform coding are two fundamental approaches to high efficiency bit reduction coding it is important to clarify the basic coding characteristics of these approaches and the differences between them in order to utilize the high efficiency coding method effectively it is important to compare them not only from the standpoint of coding performance as optimized coding schemes based on the statistics of the input picture signal but also from that of the robustness of coding performance for the variation of picture statistics to be coded this paper theoretically compares the robustness of the coding performance of dpcm having a two dimensional predictor with that of a two dimensional hadamard transform coding in an intrafield coding method of the ntsc composite signal the comparison provides theoretical evidence that transform coding is more stable than dpcm and this tendency is marked at lower bit rates such as 1 or 2 bits pel while dpcm has a higher coding performance for pictures with high autocorrelation

least squares finite element formulation for hydrodynamic modeling of semiconductor devices a least squares finite element formulation is presented for the hydrodynamic modeling of semiconductor devices the formulation results in a symmetric and positive definite algebraic system its capability of adopting equal order interpolations for all unknown variables makes it simple to formulate and easy to program the semiconductor hydrodynamic equations coupled with the poisson equation are formulated as one unified equation system in this least squares finite element scheme the developed method is examined on 1 d and 2 d deep submicron semiconductor device structures to demonstrate its capability of handling the large gradients of variables and highly nonlinear source terms in the semiconductor hydrodynamic equations

deterministic prediction in progressive coding deterministic prediction in progressive coding of images is investigated progressive coding first creates a sequence of resolution layers by beginning with an original image and reducing its resolution several times by factors of some natural number m the resultant layers are losslessly encoded beginning with the lowest resolution layer and then encoding each higher resolution image incrementally upon the previous one coding efficiency may be improved if knowledge of the rules which produced the lower resolution image of each pair is used to deterministically predict pixels of the higher so they need not be encoded given reduction rules expressing each low resolution pixel as a function of nearby high resolution pixels and previously generated low resolution pixels it is shown that finding a complete set of rules each of which deterministically predicts the value of a high resolution pixel when certain values are found in nearby low resolution pixels and previously coded high resolution pixels is np complete a recursive algorithm for solving the problem in optimal time as a depth first tree search is proposed and the characteristics of the resultant prediction process are studied

topology of nonarchimedean analytic spaces and relations to complex algebraic geometry this note surveys basic topological properties of nonarchimedean analytic spaces in the sense of berkovich including the recent tameness results of hrushovski and loeser we also discuss interactions between the topology of nonarchimedean analytic spaces and classical algebraic geometry

stabilized 3d finite elements for the numerical solution of the navier stokes equations in semiconductors abstract in this article we deal with the three dimensional numerical simulation of semiconductor devices using the viscous hydrodynamic vhd transport model a reformulation of the vhd system using entropy variables allows to end up with a quasi linear form that is symmetric and for which a stability result in form of clausius duhem inequality is proved to hold the numerical approximation of the vhd model is then performed using a time discontinuous galerkin least squares finite element formulation including a discontinuity shock capturing operator and based on a fully unstructured tetrahedral decomposition of the device domain the approach combines in a unified framework the stability and optimality features of the standard galerkin method with the ability of the scheme in effectively coping with the strong variations attained by the solution throughout the semiconductor device as is demonstrated by numerical results in the simulation of several benchmark problems subject to quite different boundary conditions

quadtree structured recursive plane decomposition coding of images the approximation of two dimensional highly correlated grey value functions can be performed using a linear model of the type f x y a bx cy the set of plane parameters pps a b c can be determined in the least squares sense for a block of size n n pixels for example starting with a block size of 2 2 pixels it is shown that the pps obey a recursive law such that the pps of a 2n 2n block can be computed recursively when only the pps of the four adjacent subblocks of size n n in the lower decomposition level are known this concept of recursive plane decomposition rpd is embedded in a quadtree data structure to obtain a new variable block size image coding algorithm that offers a high performance at a low computational cost extensive comparisons to other state of the art image coding algorithms are reported

a new flux splitting scheme a new flux splitting scheme is proposed the scheme is remarkably simple and yet its accuracy rivals and in some cases surpasses that of roe s solver in the euler and navier stokes solutions carried out in this study the scheme is robust and converges as fast as the roe splitting we propose an appropriately defined cell face advection mach number using values from the two straddling cells via associated characteristic speeds this interface mach number is then used to determine the upwind extrapolation for the convective quantities accordingly the name of the scheme is coined as the advection upstream splitting method ausm we also introduce a new pressure splitting which is shown to behave successfully yielding much smoother results than other existing pressure splittings of particular interest is the supersonic blunt body problem in which the roe scheme gives anomalous solutions the ausm produces correct solutions without difficulty for a wide range of flow conditions as well as grids

an accurate and robust flux splitting scheme for shock and contact discontinuities a flux splitting scheme ausmdv has been constructed with an aim at removing numerical dissipation of the van leer type flux vector splittings on a contact discontinuity the obtained scheme is also recognized as an improved advection upstream splitting method ausm by liou and steffen the proposed scheme has the following favorable properties accurate and robust resolution for shock and contact steady and moving discontinuities conservation of enthalpy for steady flows algorithmic simplicity and easy extension to general conservation laws such as that for chemically reacting flows a simple shock fix is presented to cure the numerical shock instability associated with the carbuncle phenomenon and an entropy fix to remove an expansion shock or glitch at the sonic point extensive numerical experiments were conducted to validate the proposed scheme for a wide range of problems and the results are compiled for comparison with several recent upwind methods

multigrid and defect correction for the steady navier stokes equations theoretical and experimental convergence results are presented for nonlinear multigrid and iterative defect correction applied to finite volume discretizations of the full steady 2d compressible navier stokes equations iterative defect correction is introduced for circumventing the difficulty in solving navier stokes equations discretized with a second or higher order accurate convective part by fourier analysis applied to a model equation an optimal choice is made for the operator to be inverted in the defect correction iteration as a smoothing technique for the multigrid method collective symmetric point gauss seidel relaxation is applied with as the basic solution technique exact newton iteration applied to a continuously differentiable first order upwind discretization of the full navier stokes equations for non smooth flow problems the convergence results obtained are already competitive with those of well established navier stokes methods for smooth flow problems the present method performs better than any standard method here first order discretization error accuracy is attained in a single multigrid cycle and second order accuracy in only one defect correction cycle the method contributes to the state of the art in efficiently computing compressible viscous flows

iterative method of movement estimation for television signals an iterative method for accurate movement estimation of television signals is analyzed the method considered is based on an iterative process applied to the conventional gradient method it is known that the gradient method has some problems both in its stability and in its accuracy for movements larger than 2 or 3 pel frame it is shown that when an iterative process is used the estimated movement converges to the actual movement rapidly and is exponentially stable it is also shown that the asymptotic accuracy of the iterative estimate is determined by the smoothness of the gradient and by the amount of the frame to frame noise some experimental results are given and compared to the theoretical result

p ranks and automorphism groups of algebraic curves let x be an irreducible complete nonsingular curve of genus g over an algebraically closed field k of positive characteristic p if 9 2 the automorphism group aut x of x is known to be a finite group and moreover its order is bounded from above by a polynomial in g of degree four stichtenoth in this paper we consider the frank zy of x and investigate relations between zy and aut x we show that zy affects the order of a sylow psubgroup of aut x 3 and that an inequality iaut x 84 9 1 9 holds for an ordinary i e zy g curve x 4

semiconductor device simulation using a viscous hydrodynamic model abstract in this article we deal with a hydrodynamic model of navier stokes ns type for semiconductors including a physical viscosity in the momentum and energy equations a stabilized finite difference scheme with upwinding based on the characteristic variables is used for the discretization of the ns equations while a mixed finite element scheme is employed for the approximation of the poisson equation a consistency result for the method is established showing that the scheme is first order accurate in both space and time we also perform a stability analysis of the numerical method applied to a linearized incompletely parabolic system in two space dimensions with vanishing viscosity a thorough numerical parametric study as a function of the heat conductivity and of the momentum viscosity is carried out in order to investigate their effect on the development of shocks in both one and two space dimensional devices

fixed and adaptive predictors for hybrid predictive transform coding hybrid predictive transform coding is studied the usual formulation is to first apply a unitary transform and then code the transform coefficients with independent dpcm coders i e the prediction is performed in the transform domain this structure is compared to spatial domain prediction where a difference signal is formed in the spatial domain and then coded by a transform coder a linear spatial domain predictor which minimizes the mean square prediction error also minimizes the mean square of each transform coefficient the two structures are equivalent if the transform domain prediction scheme is extended to a more general predictor hence the structure that gives the easiest implementation can be chosen the spatial domain structure is preferred for motion compensation and for line interlaced video signals interframe hybrid coding experiments are performed on interlaced videophone scenes using an adaptive transform coder motion compensation gives a rate reduction of 25 35 percent compared to frame difference prediction with the same mean square error the subjective advantage is even greater since the dirty window effect is not present with motion compensation it is important to perform the motion estimation with fractional pel accuracy field coding with a switched predictor using previous field in moving areas is an interesting alternative to frame coding with frame difference prediction

adaptive cosine transform coding of images in perceptual domain an adaptive cosine transform coding scheme for color images which incorporates human visual properties into the coding scheme is described it employs adaptive quantization to exploit the statistical nature of the coefficients and adaptive block distortion equalization to reduce the block edge structures inherent in block transform coding schemes results show that the subjective quality of the reconstructed images at a bit rate of 0 4 bit pixel or a compression ratio of 60 1 is very good

numerical solution of the incompressible navier stokes equations in primitive variables and velocity vorticity formulation a finite difference method is presented for the numerical solution of the navier stokes equations of motion of a viscous incompressible fluid in two dimensions in primitive variables and velocity vorticity formulation for the case of primitive variables using a staggered grid and introducing an auxiliary function of the coordinate system and considering the form of the initial equation on lines passing through the nodal point x 0 y 0 and parallel to the coordinate axes we can separate it into two parts that are finally reduced to ordinary linear differential equations one for each dimension discretization of these equations leads to a system of linear equations in n unknowns which is solved by an iterative technique and the method converges rapidly giving satisfactory results for the pressure variable we consider a pressure poisson equation with suitable neumann type boundary conditions in case of velocity vorticity formulation similar procedure is used for a collocated grid numerical results up to reynolds number 5000 confirming the accuracy of the proposed method are presented for configurations of interest such as poiseuille flow lid driven cavity flow in square domain and flow in a backward facing step in the presence of a pressure gradient

dct dst alternate transform image coding an image coding method for low bit rates is proposed it is based on alternate use of the discrete cosine transform dct and the discrete sine transform dst on image blocks this procedure achieves the removal of redundancies in the correlation between neighboring blocks as well as the preservation of continuity across the block boundaries an outline of the mathematical justification of the method assuming a certain first order gauss markov model is given the resulting coding method is then adapted to nonstationary real images by locally adapting the model parameters and improving the block classification technique simulation results are shown and compared with the performance of related previous methods namely adaptive dct and fast karhunen loeve transform fklt

a theory for multiresolution signal decomposition the wavelet representation multiresolution representations are effective for analyzing the information content of images the properties of the operator which approximates a signal at a given resolution were studied it is shown that the difference of information between the approximation of a signal at the resolutions 2 sup j 1 and 2 sup j where j is an integer can be extracted by decomposing this signal on a wavelet orthonormal basis of l sup 2 r sup n the vector space of measurable square integrable n dimensional functions in l sup 2 r a wavelet orthonormal basis is a family of functions which is built by dilating and translating a unique function psi x this decomposition defines an orthogonal multiresolution representation called a wavelet representation it is computed with a pyramidal algorithm based on convolutions with quadrature mirror filters wavelet representation lies between the spatial and fourier domains for images the wavelet representation differentiates several spatial orientations the application of this representation to data compression in image coding texture discrimination and fractal analysis is discussed