This short but systematic work demonstrates a link between Chebyshev's theorem and the explicit integration in cosmological time t and conformal time η of the Friedmann equations in all dimensions and with an arbitrary cosmological constant Λ. More precisely, it is shown that for spatially flat universes an explicit integration in t may always be carried out, and that, in the non-flat situation and when Λ is zero and the ratio w of the pressure and energy density in the barotropic equation of state of the perfect-fluid universe is rational, an explicit integration may be carried out if and only if the dimension n of space and w obey some specific relations among an infinite family. The situation for explicit integration in η is complementary to that in t. More precisely, it is shown in the flat-universe case with Λ = 0 that an explicit integration in η can be carried out if and only if w and n obey similar relations among a well-defined family which we specify, and that, when Λ = 0, an explicit integration can always be carried out whether the space is flat, closed, or open. We also show that our method may be used to study more realistic cosmological situations when the equation of state is nonlinear.Keywords: Astrophysical fluid dynamics, cosmology with extra dimensions, alternatives to inflation, initial conditions and eternal universe, cosmological applications of theories with extra dimensions, string theory and cosmology.

In the paper, the authors concisely survey and review some functions involving the gamma function and its various ratios, simply state their logarithmically complete monotonicity and related results, and find necessary and sufficient conditions for a new function involving the ratio of two gamma functions and originating from the coding gain to be logarithmically completely monotonic.

In this paper we study the integrability of the Friedmann equations, when the equation of state for the perfect-fluid universe is nonlinear, in the light of the Chebyshev theorem. A series of important, yet not previously touched, problems will be worked out which include the generalized Chaplygin gas, two-term energy density, trinomial Friedmann, Born-Infeld, two-fluid models, and Chern-Simons modified * Email address: chensx@henu.edu.cn † Email address: gwg1@damtp.cam.ac.uk ‡ Email address: yisongyang@nyu.edu 1 gravity theory models. With the explicit integration, we are able to understand exactly the roles of the physical parameters in various models play in the cosmological evolution which may also offer clues to a profound understanding of the problems in general settings. For example, in the Chaplygin gas universe, a few integrable cases lead us to derive a universal formula for the asymptotic exponential growth rate of the scale factor, of an explicit form, whether the Friedmann equation is integrable or not, which reveals the coupled roles played by various physical sectors and it is seen that, as far as there is a tiny presence of nonlinear matter, conventional linear matter makes contribution to the dark matter, which becomes significant near the phantom divide line. The Friedmann equations also arise in areas of physics not directly related to cosmology. We provide some examples ranging from geometric optics and central orbits to soap films and the shape of glaciated valleys to which our results may be applied.Keywords: Astrophysical fluid dynamics, cosmology with extra dimensions, alternatives to inflation, initial conditions and eternal universe, cosmological applications of theories with extra dimensions, string theory and cosmology.

We present an explicit integration of the kink soliton equation obtained in a recent interesting study of the classical Skyrme model where the field configurations are of a generalized hedgehog form which is of a domain-wall type. We also show that in such a reduced one-dimensional setting the first-order and second-order equations are equivalent. Consequently, in such a context, all finite-energy solitons are BPS type and precisely known.PACS numbers: 11.27.+dThe well-known study of Derrick [1] shows that for a wide class of nonlinear wave equations there exist no stable time-independent solutions of finite energy due to the conformal structure of the standard Euclidean space. In order to overcome such a difficulty one may extend the theory to contain gauge fields so that stable time-independent solutions of finite energy, known as vortices, monopoles, and instantons, exist in two, three, and four spatial dimensions [2,3,4,5]. Alternatively, Skyrme [6] showed that it is possible to introduce higher-order nonlinear terms involving derivatives, instead of gauge 1

In this work a series of methods are developed for understanding the Friedmann equation when it is beyond the reach of the Chebyshev theorem. First it will be demonstrated that every solution of the Friedmann equation admits a representation as a roulette such that information on the latter may be used to obtain that for the former. Next the Friedmann equation is integrated for a quadratic equation of state and for the Randall-Sundrum II universe, leading to a harvest of a rich collection * Email address: chensx@henu.edu.cn † Email address: gwg1@damtp.cam.ac.uk ‡ Email address: yisongyang@nyu.edu 1 of new interesting phenomena. Finally an analytic method is used to isolate the asymptotic behavior of the solutions of the Friedmann equation, when the equation of state is of an extended form which renders the integration impossible, and to establish a universal exponential growth law.Keywords: Astrophysical fluid dynamics, cosmology with extra dimensions, alternatives to inflation, initial conditions and eternal universe, cosmological applications of theories with extra dimensions, string theory and cosmology.