Improved method for finding optimal formulae for bilinear maps in a finite field

In 2012, Barbulescu, Detrey, Estibals and Zimmermann proposed a new framework to exhaustively search for optimal formulae for evaluating bilinear maps, such as Strassen or Karatsuba formulae. The main contribution of this work is a new criterion to aggressively prune useless branches in the exhaustive search, thus leading to the computation of new optimal formulae, in particular for the short product modulo X 5 and the circulant product modulo (X 5 -- 1). Moreover , we are able to prove that there is essentially only one optimal decomposition of the product of 3 x 2 by 2 x 3 matrices up to the action of some group of automorphisms

Multihomogeneous resultant formulae by means of complexes

We provide conditions and algorithmic tools so as to classify and construct the smallest possible determinantal formulae for multihomogeneous resultants arising from Weyman complexes associated to line bundles in products of projective spaces. We also examine the smallest Sylvester-type matrices, generically of full rank, which yield a multiple of the resultant. We characterize the systems that admit a purely B\'ezout-type matrix and show a bijection of such matrices with the permutations of the variable groups. We conclude with examples showing the hybrid matrices that may be encountered, and illustrations of our Maple implementation. Our approach makes heavy use of the combinatorics of multihomogeneous systems, inspired by and generalizing results by Sturmfels-Zelevinsky, and Weyman-Zelevinsky.Comment: 30 pages. To appear: Journal of Symbolic Computatio

Solitons on tori and soliton crystals

Necessary conditions for a soliton on a torus $M=\R^m/\Lambda$ to be a soliton crystal, that is, a spatially periodic array of topological solitons in stable equilibrium, are derived. The stress tensor of the soliton must be $L^2$ orthogonal to \ee, the space of parallel symmetric bilinear forms on $TM$, and, further, a certain symmetric bilinear form on \ee, called the hessian, must be positive. It is shown that, for baby Skyrme models, the first condition actually implies the second. It is also shown that, for any choice of period lattice $\Lambda$, there is a baby Skyrme model which supports a soliton crystal of periodicity $\Lambda$. For the three-dimensional Skyrme model, it is shown that any soliton solution on a cubic lattice which satisfies a virial constraint and is equivariant with respect to (a subgroup of) the lattice symmetries automatically satisfies both tests. This verifies in particular that the celebrated Skyrme crystal of Castillejo {\it et al.}, and Kugler and Shtrikman, passes both tests.Comment: 24 pages, revised version to be published. Added an existence proof for baby Skyrmions of arbitrary degree on a general two-torus for a model with general potential. Otherwise, minor improvement

Noncommutativity and theta-locality

In this paper, we introduce the condition of theta-locality which can be used as a substitute for microcausality in quantum field theory on noncommutative spacetime. This condition is closely related to the asymptotic commutativity which was previously used in nonlocal QFT. Heuristically, it means that the commutator of observables behaves at large spacelike separation like $\exp(-|x-y|^2/\theta)$, where $\theta$ is the noncommutativity parameter. The rigorous formulation given in the paper implies averaging fields with suitable test functions. We define a test function space which most closely corresponds to the Moyal star product and prove that this space is a topological algebra under the star product. As an example, we consider the simplest normal ordered monomial $:\phi\star\phi:$ and show that it obeys the theta-locality condition.Comment: LaTeX, 17 pages, no figures; minor changes to agree with published versio

Recherche automatique de formules pour calculer des formes bilinéaires

National audienceThis talk will focus on the bilinear rank problem: given a bilinear map (e.g., the product of polynomials, of finite-field elements, or of matrices), what is the smallest number of multiplications over the coefficient ring required to evaluate this function?For instance, Karatsuba's method allows one to compute the product of two linear polynomials using only three multiplications instead of four. In this talk, we give a formalization of the bilinear rank problem, which is known to be NP-hard, and propose a generic algorithm to efficiently compute exact solutions, thus proving the optimality of (or even improving) known complexity bounds from the literature.Dans cet exposé, nous nous intéressons au problème du rang bilinéaire : étant donnée une application bilinéaire (par exemple, le calcul d’un produit de polynômes, d’éléments d’un corps fini, ou encore de matrices), quel est le nombre minimal de multiplications sur le corps de base nécessaires pour évaluer cette application ?Ainsi, par exemple, la méthode de Karatsuba permet de calculer le produit de deux polynômes linéaires en seulement trois multiplications au lieu de quatre. Nous donnons dans cet exposé une formalisation du problème du rang bilinéaire, connu pour être NP-dur, et proposons un algorithme général permettant de calculer efficacement des solutions exactes, qui nous permettent ainsi de prouver l’optimalité de, voire même d’améliorer certaines bornes de la littérature