Matrice inversible | EPFL Graph Search

En mathématiques et plus particulièrement en algèbre linéaire, une matrice inversible (ou régulière ou encore non singulière) est une matrice carrée A pour laquelle il existe une matrice B de même taille n avec laquelle les produits AB et BA sont égaux à la matrice identité. :AB = BA = I_n Dans ce cas la matrice B est unique, appelée matrice inverse de A et notée B = A. Cette définition correspond à celle d’élément inversible pour la multiplication dans l’anneau des matrices carrées associé. Si les coefficients d’une matrice carrée sont pris dans un anneau commutatif K, cette matrice est inversible si et seulement si elle représente un isomorphisme de K, ce qui se traduit par un déterminant inversible. En particulier, si K est un corps commutatif tel que R ou C, l’inversibilité est caractéri

Algebraic and topological methods in combinatorics

Adrian Claudiu Valculescu

The present thesis deals with problems arising from discrete mathematics, whose proofs make use of tools from algebraic geometry and topology. The thesis is based on four papers that I have co-authored, three of which have been published in journals, and one has been submitted for publication (and also appeared as a preprint on the arxiv, and as an extendend abstract in a conference). Specifically, we deal with the following four problems: \begin{enumerate} \item We prove that if

M\in \mathbb{C}^{2\times2}

is an invertible matrix, and

B_M:\mathbb{C}^2\times\mathbb{C}^2\to\mathbb{C}

is a bilinear form

B_M(p,q)=p^TMq

, then any finite set

S

contained in an irreducible algebraic curve

C

of degree

d

\mathbb{C}^2

determines

\Omega_d(|S|^{4/3})

distinct values of

B_M

, unless

C

is a line, or is linearly equivalent to a curve defined by an equation of the form

x^k=y^l

, with

k,l\in\mathbb{Z}\backslash\\{0\\}

, and

\gcd(k,l)=1

. \item We show that if we are given

m

points and

n

lines in the plane, then the number of distinct distances between the points and the lines is

\Omega(m^{1/5}n^{3/5})

, as long as

m^{1/2}\le n\le m^2

. Also, we show that if we are given

m

points in the plane, not all collinear, then the number of distances between these points and the lines that they determine is

\Omega(m^{4/3})

. We also study three-dimensional versions of the distinct point-line distances problem. \item We prove the lower bound

\Omega(|S|^4)

on the number of ordinary conics determined by a finite point set

S

\mathbb{R}^2

, assuming that

S

is not contained in a conic, and at most

c|S|

points of

S

lie on the same line (for some $0

EPFL2017

Distinct Values Of Bilinear Forms On Algebraic Curves

Frank de Zeeuw, Adrian Claudiu Valculescu

Let B-M : C x C -> C be a bilinear form B-M(p, q) - p(T)Mq, with an invertible matrix M is an element of C-2x2. We prove that any finite set S contained in an irreducible algebraic curve C of degree d in C determines Omega(d)(vertical bar S vertical bar(4/3)) distinct values of B-M, unless C has an exceptional form. This strengthens a result of Charalambides [1] in several ways. The proof is based on that of Pach and De Zeeuw [9], who proved a similar statement for the Euclidean distance function in R. Our main motivation for this paper is that for bilinear forms, this approach becomes more natural, and should better lend itself to understanding and generalization.

Univ Calgary, Dept Math & Statistics2016

Algebraic methods for channel coding

Frédérique Oggier

This work is dedicated to developing algebraic methods for channel coding. Its goal is to show that in different contexts, namely single-antenna Rayleigh fading channels, coherent and non-coherent MIMO channels, algebraic techniques can provide useful tools for building efficient coding schemes. Rotated lattice signal constellations have been proposed as an alternative for transmission over the single-antenna Rayleigh fading channel. It has been shown that the performance of such modulation schemes essentially depends on two design parameters: the modulation diversity and the minimum product distance. Algebraic lattices, i.e., lattices constructed by the canonical embedding of an algebraic number field, or more precisely ideal lattices, provide an efficient tool for designing such codes, since the design criteria are related to properties of the underlying number field: the maximal diversity is guaranteed when using totally real number fields and the minimum product distance is optimized by considering fields with small discriminant. Furthermore, both shaping and labelling constraints are taken care of by constructing Zn-lattices. We present here the construction of several families of such n-dimensional lattices for any n, and compute their performance. We then give an upper bound on their minimal product distance, and show that with respect to this bound, existing lattice codes are optimal in the sense that no further significant coding gain could be reached. Cyclic division algebras have been introduced recently in the context of coherent Space-Time coding. These are non-commutative algebras which naturally yield families of invertible matrices, or in other words, linear codes that fulfill the rank criterion. In this work, we further exploit the algebraic structures of cyclic algebras to build Space-Time Block codes (STBCs) that satisfy the following properties: they have full rate, full diversity, non-vanishing constant minimum determinant for increasing spectral efficiency, uniform average transmitted energy per antenna and good shaping. We give algebraic constructions of such STBCs for 2, 3, 4 and 6 antennas and show that these are the only cases where they exist. We finally consider the problem of designing Space-Time codes in the noncoherent case. The goal is to construct maximal diversity Space-Time codewords, subject to a fixed constellation constraint. Using an interpretation of the noncoherent coding problem in terms of packing subspaces according to a given metric, we consider the construction of non-intersecting subspaces on finite alphabets. Techniques used here mainly derive from finite projective geometry.

EPFL2005