Purity for overconvergence

Let X \hookrightarrow[`(X)]{X \hookrightarrow \overline{X}} be an open immersion of smooth varieties over a field of characteristic p > 0 such that the complement is a simple normal crossing divisor and [`(Z)] í Z í [`(X)]{\overline{Z}\subseteq Z \subseteq \overline{X}} closed subschemes of codimension at least 2. In this paper, we prove that the canonical restriction functor between the categories of overconvergent F-isocrystals F-Isoc<sup>f</sup>(X,[`(X)]) ? F-Isoc^f(X\Z,[`(X)]\[`(Z)]){F-{\rm Isoc}^\dagger(X,\overline{X}) \longrightarrow F-{\rm Isoc}^\dagger(X{\setminus}Z, \overline{X}{\setminus}\overline{Z})} is an equivalence of categories. We also give an application of our result to the equivalence of certain categories.     

Purity conjecture for reductive groups

The following problem was posed by J.-L. Colliot-Th èléne and J.-J. Sansuc in [1, p. 124, Problem 6.4]. Given a local regular ring R and a reductive group scheme G over R determine whether the functor S → H 44-01 (S, G) satisfies the property of purity for R. In this work, we study this problem in a number of interesting particular cases. Namely, let k be a characteristic zero field, and G be one of the following algebraic groups over k: PGL _n , SL_1,A , O(q), SO(q), Spin(q), SL _n /μ _d where d divides n (here, A is a central simple k-algebra). In this paper we prove that the functor R → H _ét¹ (R, G) satisfies the property of purity for the group G and a regular local ring containing the field. In view of this result, it would appear reasonable to suggest that the aforementioned functor possesses the property of purity for an arbitrary connected reductive group G over a zero characteristic field k and an arbitrary regular local ring containing the field k. For groups of the types G ₂ and F ₄ with a trivial g₃ invariant, this conjecture has been proved in [2] and [3]. The problem and conjecture formulated above appear to be an extension of the known conjectures proposed by A. Grothendieck and J.-P. Serre (see [5, Remark 3, pp. 26–27], [6, Remark 1.11.a], and [14, Remark on p. 31]).     

Semidefinite representations for finite varieties

We consider the problem of minimizing a polynomial over a set defined by polynomial equations and inequalities. When the polynomial equations have a finite set of complex solutions, we can reformulate this problem as a semidefinite programming problem. Our semidefinite representation involves combinatorial moment matrices, which are matrices indexed by a basis of the quotient vector space ℝ[x ₁, . . . ,x _n ]/I, where I is the ideal generated by the polynomial equations in the problem. Moreover, we prove the finite convergence of a hierarchy of semidefinite relaxations introduced by Lasserre. Semidefinite approximations can be constructed by considering truncated combinatorial moment matrices; rank conditions are given (in a grid case) that ensure that the approximation solves the original problem to optimality. Supported by the Netherlands Organisation for Scientific Research grant NWO 639.032.203.     

Hall-type representations for generalised orthogroups

An orthogroup is a completely regular orthodox semigroup. The main purpose of this paper is to find a representation of a (generalised) orthogroup with band of idempotents B in terms of a fundamental (generalised) orthogroup. The latter is a subsemigroup of the Hall semigroup W _B (or of its generalisations V _B ,U _B and S _B ). We proceed in the regular case by constructing a fundamental completely regular subsemigroup \(\overline{W_{B}}\) of W _B , using two different methods. Our subsemigroup plays the role for orthogroups that W _B plays for orthodox semigroups, in that it contains a representation of every orthogroup with band of idempotents B, with kernel of the representation being μ, the greatest congruence contained in \(\mathcal{H}\) . To develop an analogous theory for classes of generalised orthogroups, that is, to extend beyond the regular case, we replace \(\mathcal{H}\) by \(\widetilde{\mathcal{H}}_{B}\) . Generalised orthogroups are then classes of weakly B-superabundant semigroups with (C). We first consider those satisfying an idempotent connected condition (IC) or (WIC). We construct fundamental weakly B-superabundant subsemigroups \(\overline{V_{B}}\) (respectively, \(\overline{U_{B}}\) ) of V _B (respectively, U _B ) with (C) and (IC) (respectively, with (C) and (WIC)) such that any weakly B-superabundant semigroup with (C) and (IC) (respectively, with (C) and (WIC)) admits a representation to \(\overline{V_{B}}\) (respectively, \(\overline{U_{B}}\) ), with kernel of the respresentation being μ _B , the greatest congruence contained in \(\widetilde{\mathcal{H}}_{B}\) . Finally, we remove the idempotent connected condition and find a representation for an arbitrary weakly B-superabundant semigroup with (C), making use of fresh technology, constructing a fundamental weakly B-superabundant subsemigroup \(\overline{S_{B}}\) of S _B , with the appropriate universal properties. We note that our results are needed in a parallel paper to complete the representation of arbitrary weakly B-superabundant semigroups with (C) as spined products of superabundant Ehresmann semigroups and subsemigroups of S _B .     

Integral representations for elliptic functions

We derive new integral representations for constituents of the classical theory of elliptic functions: the Eisenstein series, and Weierstrass' ℘ and ζ functions. The derivations proceed from the Laplace-Mellin representation of multipoles, and an elementary lemma on the summation of 2D geometric series. In addition, we present results concerning the analytic continuation of the Eisenstein series to an entire function in the complex plane, and the value of the conditionally convergent series, denoted by below, as a function of summation over increasingly large rectangles with arbitrary fixed aspect ratio.¹     

The Purity Problem for Functors with Transfers

The present paper is devoted to the purity problem for functors endowed with a structure of transfer map. Namely let R be a local regular ring of geometric type and let be a covariant functor from the category of R-algebras to Abelian groups that has a structure of transfer map. We prove that purity holds for the functor . Applying this general result to different examples of functors, we obtain new uniform proofs of known results, as well as new versions of purity.     

Purity in algebra

We investigate pure monomorphisms and pure epimorphisms in varieties of universal algebras with applications to equationally compact (= pure injective) algebras, pure projective algebras and perfect varieties. Received December 3, 2004; accepted in final form November 19, 2005. The second author was supported by the Grant Agency of the Czech Republic under the grant 201/02/0148. The hospitality of the Catholic University Louvain is gratefully acknowledged.     

Simple root flows for Hitchin representations

On a Kähler manifold there is a clear connection between the complex geometry and underlying Riemannian geometry. In some ways, this can be used to characterize the Kähler condition. While such a link is not so obvious in the non-Kähler setting, one can seek to understand extensions of these characterizations to general Hermitian manifolds. This idea has been the subject of much study from the cohomological side, however, the focus here is to address such a question from the perspective of curvature relationships. In particular, on compact manifolds the Kähler condition is characterized by the relationship that the Chern scalar curvature is equal to half the Riemannian scalar curvature. What we study here is the existence, or lack thereof, of non-Kähler Hermitian metrics for which a more general proportionality relationship between these scalar curvatures holds.     

Lattice quantization error for redundant representations

Redundant systems such as frames are often used to represent a signal for error correction, denoising and general robustness. In the digital domain quantization needs to be performed. Given the redundancy, the distribution of quantization errors can be rather complex. In this paper we study quantization error for a signal X in represented by a frame using a lattice quantizer. We completely characterize the asymptotic distribution of the quantization error as the cell size of the lattice goes to zero. We apply these results to get the necessary and sufficient conditions for the asymptotic form of the White Noise Hypothesis in the case of the pulse-code modulation scheme.     

A bilinear algorithm for sparse representations

We consider the following sparse representation problem: represent a given matrix X∈ℝ ^m×N as a multiplication X=AS of two matrices A∈ℝ ^m×n (m≤n<N) and S∈ℝ ^n×N , under requirements that all m×m submatrices of A are nonsingular, and S is sparse in sense that each column of S has at least n−m+1 zero elements. It is known that under some mild additional assumptions, such representation is unique, up to scaling and permutation of the rows of S. We show that finding A (which is the most difficult part of such representation) can be reduced to a hyperplane clustering problem. We present a bilinear algorithm for such clustering, which is robust to outliers. A computer simulation example is presented showing the robustness of our algorithm.     

Series representations for some mathematical constants

The authors apply a classical series identity involving the psi (or digamma) function with a view to deriving series representations for a number of known mathematical constants. Several closely-related consequences and results are also considered.