Exact Cosmological Solutions of Nonlinear F(R)-Gravity
We report on the cited papers refs. 1 - 18 from the following points of view: What do we exactly know about solutions when no exact solution (in the sense of "solution in closed form") is available? In which sense do these solutions possess a singularity? In which cases do conformal relations and/or dimensional reductions simplify the deduction? Furthermore, we outline some open questions worth of being studied in future research.

title: Inhomogeneous Cosmological Models with Flat Slices Generated from the EINSTEIN‐DE SITTER Universe
abstract: A family of cosmological models in considered which in a certain synchronized system of reference possess flat slices t = const. They are generated from the EINSTEIN-DE SITTER universe by a suitable transformation. Under physically reasonable presumptions these transformed models fulfil certain energy conditions. 
 
 
 
Es wird eine Familie kosmologischer Modelle betrachtet, die in einemi gewissen synchronisierten Bezugssystem flache Schichten t = const. besitzen. Sie werden mittels einer geeigneten Transformation aus dem EINSTEIN-DE SITTER-Universum erzeugt. Diese transformierten Modelle erfullen unter physikalisch sinnvollen Voraussetzungen gewisse Energie bedingungen.

title: COMMENT: Fourth-order gravity and conformal transformations

title: The Newtonian limit of fourth-order gravity
abstract: The weak-field slow-motion limit of fourth-order gravity will be discussed. Let us consider the gravitational theory defined by the Lagrangian Lg = (8πG) −1 ( R/2 + (αRijR ij + βR)l ) . (1) G is Newton’s constant, l a coupling length and α and β numerical parameters. Rij and R are the Ricci tensor and its trace. Introducing the matter Lagrangian Lm and varying Lg + Lm one obtains the field equation Eij + αHij + βGij = 8πGTij . (2) For α = β = 0 this reduces to General Relativity Theory. The explicit expressions Hij and Gij can be found in STELLE (1978). In a well-defined sense, the weak-field slow-motion limit of Einstein’s theory is just Newton’s theory, cf. DAUTCOURT (1964). In the following we consider the analogous problem for fourth order gravity eqs. (1), (2). For the special cases α = 0 (PECHLANER, SEXL(1966), POLIJEVKTOVNIKOLADZE (1967)), α + 2β = 0 (HAVAS (1977), JANKIEWICZ (1981)) 1 and α+ 3β = 0 (BORZESZKOWSKI, TREDER, YOURGRAU (1978)) this has already been done in the past. Cf. also ANANDAN (1983), where torsion has been taken into account. The slow-motion limit can be equivalently described as the limit c→ ∞, where c is the velocity of light. In this sense we have to take the limit G→ 0 while G · c and l remain constants. Then the energy-momentum tensor Tij reduces to the rest mass density ρ: Tij = δ 0 i δ 0 j ρ , (3) x = t being the time coordinate. The metric can be written as ds = (1 − 2φ)dt − (1 + 2ψ)(dx + dy + dz) . (4) Now eqs. (3) and (4) will be inserted into eq. (2). In our approach, products and time derivatives of φ and ψ can be neglected, i.e., R = 4∆ψ − 2∆φ , where ∆f = f,xx + f,yy + f,zz . Further R00 = −∆φ, H00 = −2∆R00 − ∆R and G00 = −4∆R, where l = 1. Then it holds: The validity of the 00-component and of the trace of eq. (2), R00 −R/2 + αH00 + βG00 = 8πGρ (5) and − R− 4(α+ 3β)∆R = 8πGρ , (6) imply the validity of the full eq. (2). Now, let us discuss eqs. (5) and (6) in more details: Eq. (5) reads − ∆φ−R/2 + α(2∆∆φ− ∆R) − 4β∆R = 8πGρ . (7)

title: 4D wormhole with signature change in the presence of extra dimensions
abstract: A regular vacuum solution in 5D gravity on the principal bundle with the U(1) structural group is proposed as a 4D wormhole. This solution has two null hypersurfaces where an interchange of the sign of some 5D metric components happens. For a 4D observer living on the base of this principal bundle this is a wormhole with two asymptotically flat Lorentzian (Euclidean) spacetimes connected by a Euclidean (Lorentzian) throat. The 4D Lorentzian observer sees these two null hypersurfaces as electric charges.

title: Models for Constrained Motion and d'Alembert's Principle
abstract: A theorem of Arnol'd and Gallavotti states that by a suitable potential of increasing rigidity a mechanical system with conservative forces can be constrained to a hypersurface of the configuration space and its motion approaches a solution of the reduced Lagrangian equation. In this paper the theorem will be generalized to time- and velocity-dependent forces including time-dependent constraints and proved with the aid of the method of averaging. This technique also allows us to identify the constraint force as the averaged negative gradient of the constraint potential.

title: Axiomatic Characterization of Physical Geometry
abstract: Operations with rigid bodies.- Regions and transport mappings.- The Helmhotz-Lie problem.- Characterization of Euclidean geometry.