A semialgebraic closure for commutative algebra

Let A⊂R be rings containing the rationals. In R let S be a multiplicatively closed subset such that 1∈S and 0∉S, T a preorder of R (a proper subsemiring containing the squares) such that S⊂T and I an A-submodule of R. Define ρ(I) (or ρ_S,T(I)) to be

ρ(I)={a∈R|sa^2m+t∈I^2m for some m∈N,s∈S and t∈T}.     

Finding connected components of a semialgebraic set in subexponential time

Let a semialgebraic set be defined by a quantifier-free formula with atomic subformulas of the form f_i>0, 0,1 i where the polynomials f_i[X₁,..., X_n] of degree deg (f_i)<d have absolute value of the coefficients at most 2^M. An algorithm is constructed which finds the connected components of the semialgebraic set (i.e., giving formulas for them) in a time that is polynomial inM (kd) ^ro(1). Collins' previously known method has a bound which is polynomial inM (kd) ^ro(1).Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 192, pp. 3–46, 1991.     

Kodaira dimension of holomorphic singular foliations

We introduce numerical invariants of holomorphic singular foliations under bimeromorphic transformations of surfaces. The basic invariant is a foliated version of the Kodaira dimension of compact complex manifolds.The author was supported by CNPq-Brazil in 1998 and Conseil Régional de Bourgogne in 1999.     

On the limit set at infinity of a gradient trajectory of a semialgebraic function

Given any C² semialgebraic function f defined on a non-bounded open set of Rn, we prove that the limit of the secants at infinity of a non-bounded trajectory of the gradient of f does exist. As a corollary we find a new sufficient condition to ensure the trivialisation by the gradient flow of f nearby a regular asymptotic critical value at infinity.     

Computing the dimension of a semi-algebraic set

In this paper, we consider the problem of computing the real dimension of a given semi-algebraic subset of R ^k, where R is a real closed field. We prove that the dimension k′ of a semi-algebraic set described by s polynomials of degree d in k variables can be computed in time

Hausdorff dimension of a random invariant set

The notion of random attractor for a dissipative stochastic dynamical system has recently been introduced. It generalizes the concept of global attractor in the deterministic theory. It has been shown that many stochastic dynamical systems associated to a dissipative partial differential equation perturbed by noise do possess a random attractor. In this paper, we prove that, as in the case of the deterministic attractor, the Hausdorff dimension of the random attractor can be estimated by using global Lyapunov exponents. The result is obtained under very natural assumptions. As an application, we consider a stochastic reaction-diffusion equation and show that its random attractor has finite Hausdorff dimension.     

Fractal dimension of a random invariant set

In recent years many deterministic parabolic equations have been shown to possess global attractors which, despite being subsets of an infinite-dimensional phase space, are finite-dimensional objects. Debussche showed how to generalize the deterministic theory to show that the random attractors of the corresponding stochastic equations have finite Hausdorff dimension. However, to deduce a parametrization of a ‘finite-dimensional’ set by a finite number of coordinates a bound on the fractal (upper box-counting) dimension is required. There are non-trivial problems in extending Debussche's techniques to this case, which can be overcome by careful use of the Poincaré recurrence theorem. We prove that under the same conditions as in Debussche's paper and an additional concavity assumption, the fractal dimension enjoys the same bound as the Hausdorff dimension. We apply our theorem to the 2d Navier–Stokes equations with additive noise, and give two results that allow different long-time states to be distinguished by a finite number of observations.     

Sharp dimension estimates of holomorphic functions and rigidity

Let be a complete noncompact Kähler manifold of complex dimension with nonnegative holomorphic bisectional curvature. Denote by the space of holomorphic functions of polynomial growth of degree at most on . In this paper we prove that

for all , with equality for some positive integer if and only if is holomorphically isometric to . We also obtain sharp improved dimension estimates when its volume growth is not maximal or its Ricci curvature is positive somewhere.

     

The exact hausdorff dimension of a branching set

Summary We obtain a critical function for which the Hausdorff measure of a branching set generated by a simple Galton-Watson process is positive and finite.     

Recurrence of trajectories of a stable compact invariant set of a holomorphic mapping

We show that the trajectories of all points of a stable compact invariant set of a holomorphic mapping are Poisson stable (recurrent).     

Computing the tight closure in dimension two

We study computational aspects of the tight closure of a homogeneous primary ideal in a two-dimensional normal standard-graded domain. We show how to use slope criteria for the sheaf of relations for generators of the ideal to compute its tight closure. In particular, our method gives an algorithm to compute the tight closure of three elements under the condition that we are able to compute the Harder-Narasimhan filtration. We apply this to the computation of in , where is a homogeneous polynomial.