Abstract:
It is a well-known fact that the gravitational effect of dark matter in galaxies is only noticeable when the orbital accelerations drop below $a_0 \simeq 2\times 10^{-8}$ cm s$^{-1}$ (Milgrom's Law). This peculiarity of the dynamic behaviour of galaxies was initially ascribed to a modification of Newtonian dynamics (MOND theory) and, consequently, it was used as an argument to criticize the dark matter hypothesis. In our model, warm dark matter is composed by collisionless Vlasov particles with a primordial typical velocity $\simeq 330$ km s$^{-1}$ and, consequently, they evaporated from galactic cores and reorganized in halos with a cusp at a finite distance from the galactic center (in contrast with Cold Dark Matter simulations which predict a cusp at the center of galaxies). This is confirmed by mean-field N-body simulations of the self-gravitating Vlasov dark matter particles in the potential well of the baryonic core. The rest mass of these particles, $\mu$, is determined from a kinetic theory of the early universe with a cosmological constant. We find that $\mu$ is in the range of a few keV. This result makes sterile neutrinos the best suited candidates for the main component of dark matter.

Abstract:
The continuity conditions of the radial distribution function g(r) and its close relative the cavity function y(r) are studied in the context of the Percus-Yevick (PY) integral equation for 3D square-well fluids. The cases corresponding to a well width, (w-1)*d, equal to a fraction of the diameter of the hard core, d/m, with m=1,2,3 have been considered. In these cases, it is proved that the function y(r) and its first derivative are everywhere continuous but eventually the derivative of some order becomes discontinuous at the points (n+1)d/m, n=0,1,... . The order of continuity (the highest order derivative of y(r) being continuous at a given point) is found to be proportional to n in the first case (m=1) and to 2*n in the other two cases (m=2,3), for large values of n. Moreover, derivatives of y(r) up to third order are continuous at r=d and r=w*d for w=3/2 and w=4/3 but only the first derivative is continuous for w=2. This can be understood as a non-linear resonance effect.

Abstract:
this article is an approach to constitutional recognition of the legal protection of consumers and users in the spanish system, determining the basic principles and characters, including a brief statement of the further legislative development of that recognition and their references in the european union law.

Abstract:
El presente artículo es una aproximación a las bases del reconocimiento constitucional de la protección jurídica de los consumidores y usuarios en el sistema espa ol, determinando los principios esenciales y caracteres que la presiden, incluyendo una sucinta exposición del ulterior desarrollo legislativo de aquel reconocimiento y sus referentes en el derecho de la Unión Europea que viene uniformando de manera muy enérgica el denominado derecho de consumo en los veintisiete países que en la actualidad conforman la agrupación comunitaria. This article is an approach to constitutional recognition of the legal protection of consumers and users in the Spanish system, determining the basic principles and characters, including a brief statement of the further legislative development of that recognition and their references in the European Union law.

Abstract:
We consider a fluid of $d$-dimensional spherical particles interacting via a pair potential $\phi(r)$ which takes a finite value $\epsilon$ if the two spheres are overlapped ($r<\sigma$) and 0 otherwise. This penetrable-sphere model has been proposed to describe the effective interaction of micelles in a solvent. We derive the structural and thermodynamic functions in the limit where the reduced temperature $k_BT/\epsilon$ and density $\rho\sigma^d$ tend to infinity, their ratio being kept finite. The fluid exhibits a spinodal instability at a certain maximum scaled density where the correlation length diverges and a crystalline phase appears, even in the one-dimensional model. By using a simple free-volume theory for the solid phase of the model, the fluid-solid phase transition is located.

Abstract:
A model for the radial distribution function $g(r)$ of a square-well fluid of variable width previously proposed [S. B. Yuste and A. Santos, J. Chem. Phys. {\bf 101}, 2355 (1994)] is revisited and simplified. The model provides an explicit expression for the Laplace transform of $rg(r)$, the coefficients being given as explicit functions of the density, the temperature, and the interaction range. In the limits corresponding to hard spheres and sticky hard spheres the model reduces to the analytical solutions of the Percus-Yevick equation for those potentials. The results can be useful to describe in a fully analytical way the structural and thermodynamic behavior of colloidal suspensions modeled as hard-core particles with a short-range attraction. Comparison with computer simulation data shows a general good agreement, even for relatively wide wells.

Abstract:
We address the problem of evaluating the number $S_N(t)$ of distinct sites visited up to time t by N noninteracting random walkers all initially placed on one site of a deterministic fractal lattice. For a wide class of fractals, of which the Sierpinski gasket is a typical example, we propose that, after the short-time compact regime and for large N, $S_N(t) \approx \hat{S}_N(t) (1-\Delta)$, where $\hat{S}_N(t)$ is the number of sites inside a hypersphere of radius $R [\ln (N)/c]^{1/ u}$, R is the root-mean-square displacement of a single random walker, and u and c determine how fast $1-\Gamma_t({\bf r})$ (the probability that site ${\bf r}$ has been visited by a single random walker by time t) decays for large values of r/R: $1-\Gamma_t({\bf r})\sim \exp[-c(r/R)^u]$. For the deterministic fractals considered in this paper, $ u =d_w/(d_w-1)$, $d_w$ being the random walk dimension. The corrective term $\Delta$ is expressed as a series in $\ln^{-n}(N) \ln^m \ln (N)$ (with $n\geq 1$ and $0\leq m\leq n$), which is given explicitly up to n=2. Numerical simulations on the Sierpinski gasket show reasonable agreement with the analytical expressions. The corrective term $\Delta$ contributes substantially to the final value of $S_N(t)$ even for relatively large values of N.

Abstract:
The average number $S_N(t)$ of distinct sites visited up to time t by N noninteracting random walkers all starting from the same origin in a disordered fractal is considered. This quantity $S_N(t)$ is the result of a double average: an average over random walks on a given lattice followed by an average over different realizations of the lattice. We show for two-dimensional percolation clusters at criticality (and conjecture for other stochastic fractals) that the distribution of the survival probability over these realizations is very broad in Euclidean space but very narrow in the chemical or topological space. This allows us to adapt the formalism developed for Euclidean and deterministic fractal lattices to the chemical language, and an asymptotic series for $S_N(t)$ analogous to that found for the non-disordered media is proposed here. The main term is equal to the number of sites (volume) inside a ``hypersphere'' in the chemical space of radius $L [\ln (N)/c]^{1/v}$ where L is the root-mean-square chemical displacement of a single random walker, and v and c determine how fast $1-\Gamma_t(\ell)$ (the probability that a given site at chemical distance $\ell$ from the origin is visited by a single random walker by time t) decays for large values of $\ell/L$: $1-\Gamma_t(\ell)\sim \exp[-c(\ell/L)^v]$. The parameters appearing in the first two asymptotic terms of $S_N(t)$ are estimated by numerical simulation for the two-dimensional percolation cluster at criticality. The corresponding theoretical predictions are compared with simulation data, and the agreement is found to be very good.

Abstract:
We investigate the first passage time t_{j,N} to a given chemical or Euclidean distance of the first j of a set of N>>1 independent random walkers all initially placed on a site of a disordered medium. To solve this order-statistics problem we assume that, for short times, the survival probability (the probability that a single random walker is not absorbed by a hyperspherical surface during some time interval) decays for disordered media in the same way as for Euclidean and some class of deterministic fractal lattices. This conjecture is checked by simulation on the incipient percolation aggregate embedded in two dimensions. Arbitrary moments of t_{j,N} are expressed in terms of an asymptotic series in powers of 1/ln N which is formally identical to those found for Euclidean and (some class of) deterministic fractal lattices. The agreement of the asymptotic expressions with simulation results for the two-dimensional percolation aggregate is good when the boundary is defined in terms of the chemical distance. The agreement worsens slightly when the Euclidean distance is used.

Abstract:
The distribution of times $t_{j,N}$ elapsed until the first $j$ independent random walkers from a set of $N \gg 1$, all starting from the same site, are trapped by a quenched configuration of traps randomly placed on a disordered lattice is investigated. In doing so, the cumulants of the distribution of the territory explored by $N$ independent random walkers $S_N(t)$ and the probability $\Phi_N(t)$ that no particle of an initial set of $N$ is trapped by time $t$ are considered. Simulation results for the two-dimensional incipient percolation aggregate show that the ratio between the $n$th cumulant and the $n$th moment of $S_N(t)$ is, for large $N$, (i) very large in comparison with the same ratio in Euclidean media, and (ii) almost constant. The first property implies that, in contrast with Euclidean media, approximations of order higher than the standard zeroth-order Rosenstock approximation are required to provide a reasonable description of the trapping order statistics. Fortunately, the second property (which has a geometric origin) can be exploited to build these higher-order Rosenstock approximations. Simulation results for the two-dimensional incipient percolation aggregate confirm the predictions of our approach.