bluuebunny/crossref_metadata_2025 huggingface dataset:
Locally compact groups whose conjugation representations satisfy a Kazhdan type property or have countable support
Authors: Kaniuth Eberhard , Markfort Annette
Published: November 2008

For a locally compact group G with left Haar measure and modular function δ the conjugation representation γG of G on L2(G) is defined by

f ∈ L2(G), x, y ∈ G. γG has been investigated recently (see [19, 20, 21, 24, 32, 35]). For semi-simple Lie groups, a related representation has been studied in [25]. γG is of interest not least because of its connection to questions on inner invariant means on L∞(G). In what follows suppγG denotes the support of γG in the dual space Ĝ, that is the closed subset of all equivalence classes of irreducible representations which are weakly contained in γG. The purpose of this paper is to establish relations between properties such as a variant of Kazhdan's property and discreteness or countability of supp γG and the structure of G.
Renewal theory in a random environment
Authors: Baxter Laurence A. , Li Linxiong
Published: November 2008

A random environment is modelled by an arbitrary stochastic process, the future of which is described by a σ-algebra. Renewal processes and alternating renewal processes are defined in this environment by considering the conditional distributions of random variables generated by the processes with respect to the σ-algebra. Generalizations of several of the standard limit theorems of renewal theory are derived.
On the rank of completely 0-simple semigroups
Authors: Ruškuc N.
Published: November 2008

Connected completely 0-simple semigroups are defined by a number of equivalent conditions, and a formula for the rank of these semigroups is proved. As a consequence an alternative proof of the result from [11] is given. In the case of a Rees matrix semigroupM0[G, I, Λ, P] the rank is expressed in terms of |I|, |Λ|,Gand a certain subgroup ofGdepending onP. At the end the minimal rank of all semigroupsM0[G, I, Λ, P] is found for a given groupG. Since every completely simple semigroup is connected, every result has a corollary for these semigroups.
On rational subgroups of reductive algebraic groups over integral domains
Authors: Chen Yu
Published: November 2008

Let G and G′ be reductive algebraic groups defined over infinite fields k and k′ respectively. The purpose of this paper is to show that G and G′ have isomorphic root systems if their rational subgroups G(R) and G′(R′), where R and R′ are integral domains with R ⊇ k and R′ ⊇ k′, are isomorphic to each other, except in one particular case (see Theorem 3·4). This has been proved by R. Steinberg in [6, theorem 31] for simple Chevalley groups over perfect fields. In particular, when G and G′ are semisimple and adjoint, every isomorphism between G(R) and G′(R′) induces an isomorphism between their irreducible components (see Proposition 3·3). These results imply that, when G and G′ are semisimple k-groups and when both are either simply connected or adjoint, then they are isomorphic to each other as algebraic groups if and only if their rational subgroups over an integral domain that contains k are isomorphic to each other, except in one particular case (see Corollary 3·5).
Embedded surfaces in four-manifolds, branched covers, and SO(3)-invariants
Authors: Kotschick D. , Matić G.
Published: November 2008

One of the outstanding problems in four-dimensional topology is to find the minimal genus of an oriented smoothly embedded surface representing a given homology class in a smooth four-manifold. For an arbitrary homology class in an arbitrary smooth manifold not even a conjectural lower bound is known. However, for the classes represented by smooth algebraic curves in (simply connected) algebraic surfaces, it is possible that the genus of the algebraic curve, given by the adjunction formula

is the minimal genus. This is usually called the (generalized) Thom conjecture. It is mentioned in Kirby's problem list [11] as Problem 4·36.
Unconditionally converging polynomials on Banach spaces
Authors: Gonz´lez Manuel , Gutiérrez Joaquín M.
Published: November 2008

In the study of polynomials acting on Banach spaces, the weak topology is not such a good tool as in the case of linear operators, due to the bad behaviour of the polynomials with respect to the weak convergence. For example,

is a continuous polynomial taking a weakly null sequence into a sequence having no weakly Cauchy subsequences. In this paper we show that the situation is not so bad for unconditional series. Recall that is a weakly unconditionally Cauchy series (in short a w.u.C. series) in a Banach space E if for every f ε E* we have that and is an unconditionally converging series (in short an u.c. series) if every subseries is norm convergent.
The marriage lemma and large anonymous games with countable actions
Authors: Khan M. Ali , Sun Yeneng
Published: November 2008

We give a particularly short proof of the symmetrization of Cournot-Nash equilibrium distributions in large anonymous games with countable actions. Our proof relies on the Bollobás-Varopoulos extension of the marriage lemma.
A remark on elementary contractions
Authors: Zhang Qi
Published: November 2008

LetXbe a smooth projective variety of dimensionnover the field of complex numbers. We denote byKxthe canonical bundle ofX. By Mori's theory, ifKxis not numerically effective (i.e. if there exists a curve onXwhich has negative intersection number withKx), then there exists an extremal ray ℝ+[C] onXand an elementary contractionfR: X → Y associated with ℝ+[C].fRis called a small contraction if it is bi-rational and an isomorphism in co-dimension one.
Some stability properties of c0-saturated spaces
Authors: Leung Denny H.
Published: November 2008

A Banach space is c0-saturated if all of its closed infinite-dimensional subspaces contain an isomorph of c0. In this article, we study the stability of this property under the formation of direct sums and tensor products. Some of the results are: (1) a slightly more general version of the fact that c0-sums of c0-saturated spaces are c0-saturated; (2) C(K, E) is c0-saturated if both C(K) and E are; (3) the tensor product is c0-saturated, where JH is the James Hagler space
On the asymptotic normality of Hill's estimator
Authors: Csörgő Sándor , Viharos László
Published: November 2008

LetX,X1,X2, …, be independent random variables with a common distribution functionF(x) =P{X≤x},x∈ℝ, and for eachn∈ℕ, letX1,n≤ … ≤Xn, ndenote the order statistics pertaining to the sampleX1, …,Xn. We assume that 1–F(x) =x−1/cl(x), 0 <x< ∞, wherelis some function slowly varying at infinity andc> 0 is any fixed number. The class of all such distribution functions will be denoted by.
Non-trivial links and plats with trivial Gassner matrices
Authors: Cochran Tim D.
Published: November 2008

LetBndenote the Artin braid group on ‘n-strings[ and PBnits normal subgroup consisting of all the pure braids [Bi, Mo]. These groups have been considerably scrutinized by both topologists and algebraists [BL]. One question whose answer has so far eluded us is whether or not the Gassner representationG: PBn→Mn×n(λ), into the group ofn-by-nmatrices over, is faithful (see Section 1) [Bi; ·3] [Ga]. Recently the less discriminating Burau representation B: PBn→Mn×n(Z[t±1] ) was shown to have a non-trivial kernel for each n ≥ 6 [M, LP] but these techniques have not yet yielded an element of kernel(G). This paper is a partial step in that direction.
Minimal primal ideal spaces and norms of inner derivations of tensor products of C*-algebras
Authors: Kaniuth Eberhard
Published: November 2008

An ideal I in a C*-algebra A is called primal if whenever n ≥ 2 and J1,…, Jn are ideals in A with zero product then Jk ⊆ I for at least one k. The topologized space of minimal primal ideals of A, Min-Primal (A), has been extensively studied by Archbold[3]. Very much in the spirit of Fell's work [14] it was shown in [3, theorem 5·3] (see also [5, theorem 3·4]) that if A is quasi-standard, then A is *-isomorphic to a maximal full algebra of cross-sections of Min-Primal (A). Moreover, if A is separable the fibre algebras are primitive throughout a dense subset. On the other hand, the complete regularization of the primitive ideal space of A gives rise to the space of so-called Glimm ideals of A, Glimm (A). It turned out that A is quasi-standard exactly when Min-Primal (A) and Glimm (A) coincide as sets and topologically [5, theorem 3·3].
All automorphisms of free groups with maximal rank fixed subgroups
Authors: Collins D. J. , Turner E. C.
Published: November 2008

The Scott Conjecture, proven by Bestvina and Handel [2] says that an automorphism of a free group of ranknhas fixed subgroup of rank at mostn. We characterise in Theorem A below those automorphisms that realise this maximum. It follows from this characterisation, for example, that any such automorphism has linear growth. In our paper [3], we generalised the Scott Conjecture to arbitrary free products, using Kuros rank (see Section 2 below) in place of free rank; in Theorem B, we characterise those automorphisms of a free product realising the maximum. We show that in this case the growth rate is also linear. These results extend those of [4].
Homological finiteness conditions for modules over strongly group-graded rings
Authors: Cornick Jonathan , Kropholler Peter H.
Published: November 2008

Throughout this paper, k denotes a commutative ring. We will develop a theory of homological finiteness conditions for modules over certain graded k-algebras which generalizes known theory for group algebras. The simplest of our results, Theorem A below, generalizes certain results of Aljadeff and Yi on crossed products of polycyclic-by-finite groups (cf. [1, 11]), but also applies to many other crossed products in cases where little was previously known. Before stating the results, we recall definitions of graded and strongly graded rings. Let G be a monoid. Naively, a G-graded k-algebra is a k-algebra R which admits a k-module decomposition,

in such a way that Rg Rh ⊆ for all g, h ∈ G. If R is a G-graded k-algebra and X is any subset of G, then we write Rx for the k-submodule of R supported on X; that is

Note that if H is a submonoid of G then RH is a subalgebra of R.
Existence of outgoing solutions for perturbations of and applications to the scattering matrix
Authors: Yamamoto Kazuhiro
Published: November 2008

In this paper we shall prove an existence theorem and give applications of an outgoing solution of the following problem:

where L(x, x) is a second order elliptic differential operator with a potential term q(x), is an exterior domain of ℝn (where n 2) with the C2-class boundary , k is an element of the complex plane or of a logarithmic Riemann surface, and B is either a Dirichlet boundary condition or of the form Bu = vj(x) ajk(x) ku + (x)u with the unit outer normal vector v(x) = (vl,, vn) at x.
The set of circle packing points in the Teichmüller space of a surface of finite conformal type is dense
Authors: Bowers Philip L. , Stephenson Kenneth
Published: November 2008

W. Thurston initiated interest in circle packings with his provocative suggestion at the International Symposium in Celebration of the Proof of the Bieberbach Conjecture (Purdue University, 1985) that a result of Andreev[2] had an interpretation in terms of circle packings that could be applied systematically to construct geometric approximations of classical conformal maps. Rodin and Sullivan [11] verified Thurston's conjecture in the setting of hexagonal packings, and more recently Stephenson [12] has announced a proof for more general combinatorics. Inspired by Thurston's work and motivated by the desire to discover and exploit discrete versions of classical results in complex variable theory, Beardon and Stephenson [4, 5] initiated a study of the geometry of circle packings, particularly in the hyperbolic setting. This topic is a recent example among many of the beautiful and sometimes unexpected interplay between Geometry, Topology, and Cornbinatorics that is evident in much of the topological research of the past decade, and that has its roots in the seminal work of the great geometrically-minded mathematicians – Riemann, Klein, Poincaré – of the last century. A somewhat surprising example of this interplay concerns us here; namely, the fact that the combinatorial information encoded in a simplicial triangulation of a topological surface can determine its geometry.
Localization and class groups of module categories with exactness defects
Authors: Holland D. , Wilson S. M. J.
Published: November 2008

We present a new way of forming a grothendieck group with respect to exact sequences. A ‘defect’ is attached to each non-split sequence and the relation that would normally be derived from a collection of exact sequences is only effective if the (signed) sum of the corresponding defects is zero. The theory of the localization exact sequence and, in particular, of the relative group in this sequence is developed. The (‘locally free’) class group of a module category with exactness defect is defined and an idèlic formula for this is given. The role of torsion and of torsion-free modules is investigated. One aim of the work is to enhance the locally trivial, ‘class group’, invariants obtainable for a module while keeping to a minimum the local obstructions to the definition of such invariants.
Symmetry characterizations of certain distributions. 3. Discounted additive functional and large deviations for a general finite-state Markov chain
Authors: Baxter Martin
Published: November 2008

In Baxter and Williams [1] we began a study of Abel averages,

as opposed to the oft-studied Cesàro averages In Baxter and Williams [2], hereinafter referred to as [BW2], we studied the large-deviation behaviour of these averages. In the case where X is an irreducible Markov chain on a finite state-space S = {1,…, n}, we observed that

and

where π is the invariant distribution of X. We noted that

where v is an n-vector, δ(v):= sup{Re(z): z ∈ spect(Q+ V)}, and where spect(·) denotes spectrum (here the set of eigenvalues), Q is the Q-matrix of X, and V denotes the diagonal matrix diag (vi). It is also true that the large-deviation property holds for Ct with rate function I denned on M = {(xt)i ∈ S: xi ≥ 0, Σixi = 1}.
On Markov chains with an enumerable infinity of states
Authors: Foster F. G.
Published: November 2008

Consider the infinite system of linear ‘inequations’,

in which the coefficients pij have the two properties

We define as properly divergent (p.d.) a solution, {xi} (i = 0, 1, 2, …), such that

and such that |xi| < ∞ for i < ∞. Since {k + xi} is also a solution for any non-negative constant k, there is no loss in generality in assuming (as we always shall) in place of this last condition that

We are concerned with conditions for the existence of p.d. solutions.
An example of a divergent perturbation expansion in field theory
Authors: Hurst C. A.
Published: November 2008

The convergence of the perturbation expansion for the S-matrix in interaction representation of a three-boson contact interaction is investigated. A lower bound is obtained for the integrals corresponding to irreducible graphs when the total rest mass of the system is insufficient for bare particles to be created. It is shown that in this case the perturbation expansion cannot converge no matter what value the coupling constant has. A discussion is given of the bearing of this result on the general problem of the convergence of perturbation expansions for the S-matrix in renormalized field theories.
Type M for quasi-Hausdorff matrices
Authors: Rhoades B. E.
Published: November 2008

Let A = (ank) denote an infinite matrix. Then A is said to be of type M if the conditions

imply αn = 0(n = 0,1,2,…).
Multi-valued mappings and Lefschetz fixed point theorems
Authors: Powers Michael J.
Published: November 2008

By a multi-valued map from a space X to a space Y we mean a map which assigns to each point x in X a non-empty subset F(x) of Y. When X = Y, a point x in X is a fized point for F if x is in F(x).
A 3-manifold which is not a product
Authors: Epstein D. B. A. , Scott G. P.
Published: November 2008

The purpose of this note is to construct a non-compact 3-manifold M with the following properties:

(i) M is orientable and irreducible

(ii) ∂M is connected and compact and its inclusion in M is a homotopy equivalence

(iii) M is not homeomorphic to ∂M × R+.

(Throughout this paper, all manifolds will be PL and all embeddings will be PL and locally flat.)
A conjecture of Fejes Tóth on saturated systems of circles
Authors: Dumir Vishwa Chander , Khassa Dharam Singh
Published: November 2008

A family ℳ of closed circular discs in the plane is called a saturated family or a saturated system of circles if (i) the infimum r of the radii of the discs in ℳ is positive and (ii) every closed disc of radius r in the plane intersects at least one disc in ℳ. For a saturated family ℳ, we denote by S the point-set union of the interiors of the members of ℳ and by S(l) the part of S inside the circular disc of radius l centred at the origin. We define the lower density ρℳ of the saturated family ℳ as

where V(S(l)) denotes the Lebesgue measure of the set S(l).
Determination of the zeros of a cross-product Bessel function
Authors: Salchev L. Z. , Popov V. B.
Published: November 2008

In many mechanical and other problems the following equation

is reached, where Jν(α) and Yν(α) are Bessel functions of the first and second kind of any real order ν and β is a positive parameter.

For example, equation (1) is reached in the case of determining the critical load Pcr, for a simply supported strut with a variable inertia moment by a power law, where the power m is any real number.
The Uniqueness of a Certain Trigonometric Integral
Authors: Offord A. C.
Published: November 2008

Verblunsky has recently proved the following theorem:

Theorem A. Suppose that

for every positive σ. Write

and suppose that J (σ, x) is bounded in σ, x for σ > 0 and all x. Then

exists almost everywhere, and

almost everywhere.
On the Effective Temperatures of Extended Photospheres
Authors: Chandrasekhar S.
Published: November 2008

For material stratified in parallel planes in local thermodynamical equilibrium we have Milne's well-known result that

where πF is the constant net integrated flux of radiation, τ is the optical depth and B is the “ergiebigkeit” which is related to the temperature T by the relation

σ being the Stefan-Boltzmann constant. If we define the effective temperature by the relation

we have from (1)

where T1 is the temperature ar τ = 1. In this note we establish a similar result for extended photospheres where the curvature of the outer layers is properly taken into account.
The Statistics of the Hydrogen-Palladium System
Authors: Lacher J. R.
Published: November 2008

In a recent paper, the solubility of hydrogen in palladium was discussed. The statistical theory employed was essentially the same as that given by Fowler† for critical adsorption, the only modification necessary being to take into account the dissociation of the hydrogen molecules on absorption. In the present paper Peierls' theory‡ is applied to the same problem in order to enable a comparison to be made between the theories. According to both theories we must postulate that the absorbed (adsorbed) particles go into a definite number of potential energy holes and that the heat of absorption increases as the number of holes filled increases. Fowler uses the Bragg and Williams type of approximation to determine the energy of the particles in the holes; while Peierls supposes that there is an interaction energy V for each pair of neighbouring adsorbed particles and that all those arrangements of the particles in the holes having the same number of nearest neighbours have the same energy. The calculation of the dependence of the energy on the arrangements is approximated to by a method due to Bethe.
Theoretical Pearls
Authors: Barendsen Erik
Published: November 2008

For numeral systems in untyped λ-calculus the definability of a successor, a predecessor and a test for zero implies the definability of all recursive functions on that system. Towards a disproof of the converse statement, H. P. Barendregt and the author constructed a numeral system consisting of unsolvable λ-terms, being adequate for unary functions. Then, independently, B. Intrigila found an analogous system for all computable functions.
Functional Pearl: On generating unique names
Authors: Augustsson Lennart , Rittri Mikael , Synek Dan
Published: November 2008

And Joktan begat Almodad, and Sheleph, and Hazarmaveth, and Jerah, and Handoram, and Uzal, and Diklah, and Obal, and Abimael, and Sheba, and Ophir, and Havilah, and Jobab: all these were the sons of Joktan.

— Genesis 10:26–29
Untyped strictness analysis
Authors: Ernoult Christine , Mycroft Alan
Published: November 2008

We re-express Hudak and Young's higher-order strictness analysis for the untyped λ-calculus in a conceptually simpler and more semantically-based manner. We show our analysis to be a sound abstraction of Hudak and Young's which is also complete in a sense we make precise.
The ins and outs of Clean I/O
Authors: Achten Peter , Plasmeijer Rinus
Published: November 2008

Functional programming languages have banned assignment because of its undesirable properties. The reward of this rigorous decision is that functional programming languages are side-effect free. There is another side to the coin: because assignment plays a crucial role in Input/Output (I/O), functional languages have a hard time dealing with I/O. Functional programming languages have therefore often been stigmatised as inferior to imperative programming languages because they cannot deal with I/O very well. In this paper, we show that I/O can be incorporated in a functional programming language without loss of any of the generally accepted advantages of functional programming languages. This discussion is supported by an extensive account of the I/O system offered by the lazy, purely functional programming language Clean. Two aspects that are paramount in its I/O system make the approach novel with respect to other approaches. These aspects are the technique of explicit multiple environment passing, and the Event I/O framework to program Graphical User I/O in a highly structured and high-level way. Clean file I/O is as powerful and flexible as it is in common imperative languages (one can read, write, and seek directly in a file). Clean Event I/O provides programmers with a high-level framework to specify complex Graphical User I/O. It has been used to write applications such as a window-based text editor, an object based drawing program, a relational database, and a spreadsheet program. These graphical interactive programs are completely machine independent, but still obey the look-and-feel of the concrete window environment being used. The specifications are completely functional and make extensive use of uniqueness typing, higher-order functions, and algebraic data types. Efficient implementations are present on the Macintosh, Sun (X Windows under Open Look) and PC (OS/2).
Optimal purely functional priority queues
Authors: Brodal Gerth Stølting , Okasaki Chris
Published: November 2008

Brodal recently introduced the first implementation of imperative priority queues to supportfindMin, insertandmeldinO(1) worst-case time, anddeleteMininO(logn) worst-case time. These bounds are asymptotically optimal among all comparison-based priority queues. In this paper, we adapt Brodal's data structure to a purely functional setting. In doing so, we both simplify the data structure and clarify its relationship to the binomial queues of Vuillemin, which support all four operations inO(logn) time. Specifically, we derive our implementation from binomial queues in three steps: first, we reduce the running time ofinserttoO(1) by eliminating the possibility of cascading links; second, we reduce the running time offindMintoO(1) by adding a global root to hold the minimum element; and finally, we reduce the running time ofmeldtoO(1) by allowing priority queues to contain other priority queues. Each of these steps is expressed using ML-style functors. The last transformation, known as data-structural bootstrapping, is an interesting application of higher-order functors and recursive structures.
Numerical analysis of dynamical systems
Authors: Stuart Andrew M.
Published: November 2008

This article reviews the application of various notions from the theory of dynamical systems to the analysis of numerical approximation of initial value problems over long-time intervals. Standard error estimates comparing individual trajectories are of no direct use in this context since the error constant typically grows like the exponential of the time interval under consideration.

Instead of comparing trajectories, the effect of discretization on various sets which are invariant under the evolution of the underlying differential equation is studied. Such invariant sets are crucial in determining long-time dynamics. The particular invariant sets which are studied are equilibrium points, together with their unstable manifolds and local phase portraits, periodic solutions, quasi-periodic solutions and strange attractors.

Particular attention is paid to the development of a unified theory and to the development of an existence theory for invariant sets of the underlying differential equation which may be used directly to construct an analogous existence theory (and hence a simple approximation theory) for the numerical method.
The New qd Algorithms
Authors: Parlett Beresford N.
Published: November 2008

Let us think about ways to find both eigenvalues and eigenvectors of tridiagonal matrices. An important special case is the computation of singular values and singular vectors of bidiagonal matrices. The discussion is addressed both to specialists in matrix computation and to other scientists whose main interests lie elsewhere. The reason for hoping to communicate with two such diverse sets of readers at the same time is that the content of the survey, though of recent origin, is quite elementary and does not demand familiarity with much beyond triangular factorization and the Gram-Schmidt process for orthogonalizing a set of vectors. For some readers the survey will cover familiar territory but from a novel perspective. The justification for presenting these ideas is that they lead to new variations of current methods that run a lot faster while achieving greater accuracy.
Eigenvalue optimization
Authors: Lewis Adrian S. , Overton Michael L.
Published: November 2008

Optimization problems involving eigenvalues arise in many different mathematical disciplines. This article is divided into two parts. Part I gives a historical account of the development of the field. We discuss various applications that have been especially influential, from structural analysis to combinatorial optimization, and we survey algorithmic developments, including the recent advance of interior-point methods for a specific problem class: semidefinite programming. In Part II we primarily address optimization of convex functions of eigenvalues of symmetric matrices subject to linear constraints. We derive a fairly complete mathematical theory, some of it classical and some of it new. Using the elegant language of conjugate duality theory, we highlight the parallels between the analysis of invariant matrix norms and weakly invariant convex matrix functions. We then restrict our attention further to linear and semidefinite programming, emphasizing the parallel duality theory and comparing primal-dual interior-point methods for the two problem classes. The final section presents some apparently new variational results about eigenvalues of nonsymmetric matrices, unifying known characterizations of the spectral abscissa (related to Lyapunov theory) and the spectral radius (as an infimum of matrix norms).
Numerical solution of highly oscillatory ordinary differential equations
Authors: Petzold Linda R. , Jay Laurent O. , Yen Jeng
Published: November 2008

One of the most difficult problems in the numerical solution of ordinary differential equations (ODEs) and in differential-algebraic equations (DAEs) is the development of methods for dealing with highly oscillatory systems. These types of systems arise, for example, in vehicle simulation when modelling the suspension system or tyres, in models for contact and impact, in flexible body simulation from vibrations in the structural model, in molecular dynamics, in orbital mechanics, and in circuit simulation. Standard numerical methods can require a huge number of time-steps to track the oscillations, and even with small stepsizes they can alter the dynamics, unless the method is chosen very carefully.
Radiation boundary conditions for the numerical simulation of waves
Authors: Hagstrom Thomas
Published: November 2008

We consider the efficient evaluation of accurate radiation boundary conditions for time domain simulations of wave propagation on unbounded spatial domains. This issue has long been a primary stumbling block for the reliable solution of this important class of problems. In recent years, a number of new approaches have been introduced which have radically changed the situation. These include methods for the fast evaluation of the exact nonlocal operators in special geometries, novel sponge layers with reflectionless interfaces, and improved techniques for applying sequences of approximate conditions to higher order. For the primary isotropic, constant coefficient equations of wave theory, these new developments provide an essentially complete solution of the numerical radiation condition problem.
Numerical methods in tomography
Authors: Natterer Frank
Published: November 2008

In this article we review the image reconstruction algorithms used in tomography. We restrict ourselves to the standard problems in the reconstruction of function from line or plane integrals as they occur in X-ray tomography, nuclear medicine, magnetic resonance imaging, and electron microscopy. Nonstandard situations, such as incomplete data, unknown orientations, local tomography, and discrete tomography are not dealt with. Nor do we treat nonlinear tomographic techniques such as impedance, ultrasound, and near-infrared imaging.
How paleosols influence groundwater flow and arsenic pollution: A model from the Bengal Basin and its worldwide implication
Authors: McArthur J. M. , Ravenscroft P. , Banerjee D. M. , Milsom J. , Hudson‐Edwards K. A. , Sengupta S. , Bristow C. , Sarkar A. , Tonkin S. , Purohit R.
Published: November 2008

In the Bengal Basin, the land surface exposed during the last lowstand of sea level around 20 ka, and now buried by Holocene sediment, is capped by an effectively impermeable clay paleosol that we term the Last Glacial Maximum paleosol (LGMP). The paleosol strongly affects groundwater flow and controls the location of arsenic pollution in the shallow aquifers of our study site in southern West Bengal and, by implication, in shallow aquifers across the Bengal Basin and As‐polluted deltaic aquifers worldwide. The presence of the LGMP defines paleointerfluvial areas; it is absent from paleochannel areas. A paleosol model of pollution proposed here predicts that groundwater in paleochannels is polluted by arsenic, while that beneath paleointerfluvial areas is not: paleointerfluvial aquifers are unpolluted because they are protected by the LGMP from downward migration of arsenic and from downward migration of organic matter that drives As‐pollution via reductive dissolution of As‐bearing iron oxyhydroxides. Horizontal groundwater flow carries arsenic from paleochannels toward paleointerfluvial aquifers, in which sorption of arsenic minimizes the risk of pollution.
Kindlins: essential regulators of integrin signalling and cell–matrix adhesion
Authors: Larjava Hannu , Plow Edward F , Wu Chuanyue
Published: November 2008

Integrin‐mediated cell–ECM (extracellular matrix) adhesion is a fundamental process that controls cell behaviour. For correct cell–ECM adhesion, both the ligand‐binding affinity and the spatial organization of integrins must be precisely controlled; how integrins are regulated, however, is not completely understood. Kindlins constitute a family of evolutionarily conserved cytoplasmic components of cell–ECM adhesions that bind to β‐integrin cytoplasmic tails directly and cooperate with talin in integrin activation. In addition, kindlins interact with many components of cell–ECM adhesions—such as migfilin and integrin‐linked kinase—to promote cytoskeletal reorganization. Loss of kindlins causes severe defects in integrin signalling, cell–ECM adhesion and cytoskeletal organization, resulting in early embryonic lethality (kindlin‐2), postnatal lethality (kindlin‐3) and Kindler syndrome (kindlin‐1). It is therefore clear that kindlins, together with several other integrin‐proximal proteins, are essential for integrin signalling and cell–ECM adhesion regulation.
Radical Surgery and Reoperation in Supratentorial Malignant Glial Tumors
Authors: Daneyemez M. , Gezen F. , Çanakçi Z. , Kahraman S.
Published: November 2008

The treatment modalities for gliomas are still questioning and searching. We reviewed the effect of the extent of surgical resection and reoperation on the length and quality of survival in 152 consecutive patients who underwent operation for supratentorial gliomas at GATA Neurosurgery clinic between 1985 to 1995.Seventy-two patients (50%) had glioblastoma multiforme (GBM), and 48 patients (33%) had anaplastic astrocytoma (AA). Gross total resection was achieved in 70 cases (49%), subtotal resection was performed in 60 cases (42%), and biopsy was carried out in 14 cases (9%). Thirty-two patients were reoperated for recurrency and the median interval between the first operation and reoperation was 9.5 months in glioblastoma multiforme, and 11.7 months in anaplastic astrocytoma. The resection groups were compared for age, sex, preoperative and postoperative Karnofsky rating, tumor location, postoperative radiation therapy, and chemotherapy, and survival according to multivariate analysis. Preoperative Karnofsky rating and surgical resection type were the most important factors related to survival after operation or reoperation. The gross total resection group lived longer than the subtotal resection group by life table analysis. Median survival of GBM was 76 weeks in gross total resection group, and 33 months in AA group with total resection (p < 0.001). Preoperative Karnofsky scores had a statistically significant effect on the quality of life and survival after operation and reoperation in all cases (p = 0.005). Radical surgery and reoperation also improve quality and length of life in selective malignant supratentorial gliomas.
Medikamentöse Therapie der benignen Prostatahyperplasie
Authors: Fleig P. W. , Thüroff J. W.
Published: November 2008

Die medikamentöse Therapie der BPH stellt eine wichtige Therapieoption im klinischen Stadium I dar. Die Bewertung der zur Verfügung stehenden Medikamente setzt nicht nur eine kritische Analyse des Wirkungs/Nebenwirkungs-Profils anhand plazebokontrollierter Studien voraus, sondern erfordert darüber hinaus eine Kosten-Nutzen-Analyse. Entsprechend ihres Wirkungsmechanismus lassen sich die Präparate in Antiandrogene, Antiöstrogene, α-Rezeptoren-Blocker und Phytotherapeutika einteilen. Auf der Grundlage einer Literaturanalyse werden die verfügbaren alternativen Pharmaka bezüglich ihres Wirkungsansatzes differenziert, hinsichtlich ihrer Wirksamkeit auf subjektive Symptome und objektivierte Symptomenscores und Obstruktionsparameter analysiert, sowie die spezifischen Nebenwirkungen nach Art und Häufigkeit dargestellt.
Computertomographische Befunde bei Milzruptur, subkapsulärem Milzhämatom und perisplenitischem Abszeß
Authors: Böttger E. , Semerak M. , Jaschke W.
Published: November 2008

Die computertomographischen Kriterien der Milzruptur, frischer oder älterer Milzhämatome (Milzpseudozysten) und des perisplenitischen bzw. subphrenischen Abszesses werden anhand von drei eigenen Beobachtungen herausgearbeitet. Es zeigt sich, daß bei den genannten Erkrankungen alle präoperativ relevanten Fragen durch die Computertomographie geklärt werden können, so daß auf die Durchführung der Arteriographie verzichtet werden kann. Eine computertomographische Differentialdiagnose zwischen alten Hämatomen und perisplenitischen Abszessen ist nur unter Einbeziehung anamnestischer und klinischer Daten möglich.
The Use of Intraoperative Image-Guided Surgical Techniques for Reconstruction of Orbital and Zygomatic Deformities
Authors: Frodel John L. , Pacheco Elba
Published: November 2008

New Concepts in Surgery of the Facial Skeleton

Secondary orbital and periorbital skeletal reconstruction remains a difficult challenge. Recent use of intraoperative CT image guided surgery techniques have allowed an improved operator confidence in such reconstructions. In this review, cases illustrating the use of image guided surgery techniques in patients with fixed, secondary orbital and zygomatic deformities will be demonstrated.
The photodissociation mechanisms of acrylonitrile: Ab initio calculations on reaction channels and surface intersections
Authors: Du Wei-Na , Luo Cheng , Li Ze-Sheng
Published: November 2008

The dissociations of CH2CHCN into CH2CH+CN and CH2C+HCN in the S0, T1, and π12πC≡N∗ (definitions of π orbitals can refer to computational details) states, have been explored at the complete active space self-consistent field level of theory employing the Dunning correlation consistent triple-zeta basis set. The lowest energy points of the surface crossing seams have been searched. Two conical intersections, from π1C≡Nπ1∗ to π12π1∗ (CI1) and from π12π1∗ to S0 (CI2), and one intersystem crossing point (T1/S0) have been located. The energies of all critical points have been recomputed with the multiconfigurational second-order perturbation method. At each conical intersection, derivative coupling and unscaled gradient difference vectors have been analyzed to determine the relaxation channels that the molecule may evolve in after nonradiative decay. Once the molecule is photoexcited to the π12π1∗ or π1C≡Nπ1∗ state, it would relax along the similar pathway: funneling through CI1 and then CI2, and finally populate the ground state. Our results show that upon 193 nm photoexcitation, the most probable reaction channel is the ground-state HCN elimination following radiationless decays from excited states through surface crossings, which consists with experimental results J. Chem. Phys. 108, 5784 (1998). The investigated dissociation channels on the π12πC≡N∗ surface, which are inaccessible upon 193 nm photoexcitation, may provide information for reactions induced by higher energy excitations.
Potential distributions from electroacoustic polarization sources
Authors: Nguyen Cuong K. , Gusev Vitalyi E. , Diebold Gerald J.
Published: November 2008

Imaging with the ultrasonic vibration potential is based on recording current from a time varying polarization generated by a colloidal or ionic solution irradiated by ultrasound. Here, potential distributions between two parallel grounded electrodes are calculated for colloidal infinite layers, upright cylinders, and spheres. The calculations are valid for any source of externally imposed time varying polarization that gives rise to an oscillating electric displacement vector between the electrodes. The results of the calculations confirm several qualitative characteristics of the frequency dependence of the current reported in imaging experiments.
Environment-assisted quantum walks in photosynthetic energy transfer
Authors: Mohseni Masoud , Rebentrost Patrick , Lloyd Seth , Aspuru-Guzik Alán
Published: November 2008

Energy transfer within photosynthetic systems can display quantum effects such as delocalized excitonic transport. Recently, direct evidence of long-lived coherence has been experimentally demonstrated for the dynamics of the Fenna–Matthews–Olson (FMO) protein complex [Engel et al., Nature (London) 446, 782 (2007)]. However, the relevance of quantum dynamical processes to the exciton transfer efficiency is to a large extent unknown. Here, we develop a theoretical framework for studying the role of quantum interference effects in energy transfer dynamics of molecular arrays interacting with a thermal bath within the Lindblad formalism. To this end, we generalize continuous-time quantum walks to nonunitary and temperature-dependent dynamics in Liouville space derived from a microscopic Hamiltonian. Different physical effects of coherence and decoherence processes are explored via a universal measure for the energy transfer efficiency and its susceptibility. In particular, we demonstrate that for the FMO complex, an effective interplay between the free Hamiltonian evolution and the thermal fluctuations in the environment leads to a substantial increase in energy transfer efficiency from about 70% to 99%.
A compact all-fiber displacement interferometer for measuring the foil velocity driven by laser
Authors: Weng Jidong , Wang Xiang , Ma Yun , Tan Hua , Cai Lingcang , Li Jianfeng , Liu Cangli
Published: November 2008

A compact all-fiber displacement interferometer (AFDI) system, working at 1550 nm, has been developed and tested, and its working fundamentals will be introduced in this letter. In contrast with other models of fiber-optic velocity interferometer system, AFDI adopts a single-mode optic fiber pigtail as the detect head, diameter of which is only 1 mm, to collect directly the reflect laser beam from the moving surface, which makes this instrument have some unique advantages in observing the point movements of a small flyer. Preliminary experiments using this instrument to measure the velocity history of a small aluminum thin foil driven by a nanosecond pulse laser were conducted successfully, the precise velocity history profile deduced from the sharp interference fringes and the nanometer resolution in displacement gives an eloquent proof of its eminent abilities. The field depth (∼2 mm) of our AFDI is a little smaller than the DISAR [Weng et al., Appl. Phys. Lett. 89, 111101 (2006)] system, but its compact structure makes it much convenient to operate. Further applications for multipoints velocity history measurements of small targets are under consideration.
Influence of doping on the photorefractive properties of a polymer-dispersed liquid crystal
Authors: Redler Andreas , Kitzerow Heinz-S.
Published: November 2008

The photorefractive effect is studied in a photoconducting polysiloxane containing small droplets of a low molar mass liquid crystal. In order to vary the trap density for charge carriers, the system is doped with tetramethyl-phenylendiamine. The measured energy transfer from one beam to the other (two-beam coupling) indicates a high gain coefficient, which is enhanced by adding small concentrations of the dopant. Measurements of the phase and the amplitude of the refractive index modulation confirm that this behavior can be attributed to an increase in the trap density.
All-optical switching of holographic gratings made of polymer-liquid-crystal-polymer slices containing azo-compounds
Authors: De Sio Luciano , Veltri Alessandro , Umeton Cesare , Serak Svetlana , Tabiryan Nelson
Published: November 2008

We present the observation of an all-optical switching effect that takes place when a light beam of suitable wavelength irradiates a “layered” structure made of polymer slices alternated to films of well aligned liquid crystal (LC) which contain also a small percentage of azo-LC molecules. The simple and reliable fabrication procedure exploits a holographic UV curing technique. The observed switching effect takes place in samples that exhibit diffraction efficiency as high as 85% and is induced in less than 0.5 s by an unfocused pump beam of 245 mW/cm2 power density.
Ocean acidification causes bleaching and productivity loss in coral reef builders
Authors: Anthony K. R. N. , Kline D. I. , Diaz-Pulido G. , Dove S. , Hoegh-Guldberg O.
Published: November 2008

Ocean acidification represents a key threat to coral reefs by reducing the calcification rate of framework builders. In addition, acidification is likely to affect the relationship between corals and their symbiotic dinoflagellates and the productivity of this association. However, little is known about how acidification impacts on the physiology of reef builders and how acidification interacts with warming. Here, we report on an 8-week study that compared bleaching, productivity, and calcification responses of crustose coralline algae (CCA) and branching (Acropora) and massive (Porites) coral species in response to acidification and warming. Using a 30-tank experimental system, we manipulated CO2levels to simulate doubling and three- to fourfold increases [Intergovernmental Panel on Climate Change (IPCC) projection categories IV and VI] relative to present-day levels under cool and warm scenarios. Results indicated that high CO2is a bleaching agent for corals and CCA under high irradiance, acting synergistically with warming to lower thermal bleaching thresholds. We propose that CO2induces bleaching via its impact on photoprotective mechanisms of the photosystems. Overall, acidification impacted more strongly on bleaching and productivity than on calcification. Interestingly, the intermediate, warm CO2scenario led to a 30% increase in productivity inAcropora, whereas high CO2lead to zero productivity in both corals. CCA were most sensitive to acidification, with high CO2leading to negative productivity and high rates of net dissolution. Our findings suggest that sensitive reef-building species such as CCA may be pushed beyond their thresholds for growth and survival within the next few decades whereas corals will show delayed and mixed responses.
A placenta-specific receptor for the fusogenic, endogenous retrovirus-derived, human syncytin-2
Authors: Esnault Cécile , Priet Stéphane , Ribet David , Vernochet Cécile , Bruls Thomas , Lavialle Christian , Weissenbach Jean , Heidmann Thierry
Published: November 2008

Syncytin-2
is an envelope gene from the human endogenous retrovirus FRD (HERV-FRD) co-opted by an ancestral primate host, conserved in evolution over >40 Myr, specifically expressed in the placenta, and with a cell–cell fusogenic activity likely contributing to placenta morphogenesis. Here, using the GeneBridge4 human/Chinese hamster radiation hybrid panel, we mapped and identified the human receptor for syncytin-2. This receptor—namely Major Facilitator Superfamily Domain Containing 2 (MFSD2)—belongs to a large family of presumptive carbohydrate transporters with 10–12 membrane-spanning domains, is located at chromosomal position 1p34.2, and is conserved in evolution. An expression vector for MFSD2 confers fusogenicity to otherwise insusceptible cells upon trans-fection of
syncytin-2
. It also confers infectivity to syncytin-2 pseudotypes, consistent with this protein being the receptor for the ancestrally acquired HERV-FRD family of endogenous retroviruses. At variance with the human gene, neither mouse nor rat
MFSD2
can mediate membrane fusion, which is consistent with the fact that the envelope-derived
syncytin
genes co-opted by rodents during evolution are not orthologous to the human
syncytin
genes. Remarkably, a real-time quantitative RT-PCR analysis of
MFSD2
in various human tissues demonstrates specific expression in the placenta, as well as in the human BeWo choriocarcinoma cell line, which discloses enhancement of receptor expression upon induction by forskolin of cell–cell fusion and syncytium formation. In situ hybridization of human placental tissue using an
MFSD2
-specific probe further unambiguously demonstrates receptor expression at the level of the syncytiotrophoblast, again consistent with a role in placenta morphogenesis.