Nombre algébrique

Un nombre algébrique, en mathématiques, est un nombre complexe solution d'une équation polynomiale à coefficients dans le corps \Q des rationnels (autrement dit racine d'un polynôme non nul à coefficients rationnels). Les nombres entiers et rationnels sont algébriques, ainsi que toutes les racines de ces nombres. Les nombres complexes qui ne sont pas algébriques, comme π et e (théorème de Lindemann-Weierstrass), sont dits transcendants. L'étude de ces nombres, de leurs polynômes minimaux et des corps qui les contiennent fait partie de la théorie de Galois. Exemples

Tout nombre rationnel a est algébrique, car il est solution de l'équation x – a = 0.
Un nombre irrationnel peut être ou non algébrique. Par exemple ou ()/2 sont algébriques (car ils sont solutions de x – 2 = 0 et 8x – 3 = 0, respectivement), tandis que le nombre e est transcendant (c'est-à-dire non algébrique).
L'unité imaginaire i est algébrique, car il est solution de l'équation x + 1 =

Algebraic Nahm transform for parabolic Higgs bundles on P^1

Szilárd Szabó

We formulate the Nahm transform in the context of parabolic Higgs bundles on P^1 and extend its scope in completely algebraic terms. This transform requires parabolic Higgs bundles to satisfy an admissibility condition and allows Higgs fields to have poles of arbitrary order and arbitrary behavior. Our methods are constructive in nature and examples are provided. The extended Nahm transform is established as an algebraic duality between moduli spaces of parabolic Higgs bundles. The guiding principle behind the construction is to investigate the behavior of spectral data near the poles of Higgs fields.

2014

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

Algebraic methods for channel coding

Frédérique Oggier

EPFL2005

Algebraic Nahm transform for parabolic Higgs bundles on P^1

Hermitian lattices and bounds in K-theory of algebraic integers

Algebraic methods for channel coding

Algebraic Nahm transform for parabolic Higgs bundles on P^1

Hermitian lattices and bounds in K-theory of algebraic integers

Algebraic methods for channel coding