Nous montrons que la fonction de Christoffel (CF) se factorise en le produit de deux fonctions de Christoffel dont une est celle de la marginale et l’autre est liée à la probabilité conditionnelle. La démonstration utilise une propriété apparemment ignorée (mais intéressante en soi), qui stipule que tout polynôme qui est somme de carrés est aussi la fonction de Christoffel d’une forme linéaire (représentée par une mesure dans le cas univarié). Il en va de même pour le cône convexe des polynômes positifs sur un ensemble basique semi-algébrique. Cette interprétation de la fonction de Christoffel fournit un pont supplémentaire entre l’optimisation polynomiale et les polynômes orthogonaux.
We show that the Christoffel function (CF) factorizes (or can be disintegrated) as the product of two Christoffel functions, one associated with the marginal and the another related to the conditional distribution, in the spirit of “the CF of the disintegration is the disintegration of the CFs”. In the proof one uses an apparently overlooked property (but interesting in its own) which states that any sum-of-squares polynomial is the Christoffel function of some linear form (with a representing measure in the univariate case). The same is true for the convex cone of polynomials that are positive on a basic semi-algebraic set. This interpretation of the CF establishes another bridge between polynomials optimization and orthogonal polynomials.
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@article{CRMATH_2022__360_G9_1071_0,
author = {Lasserre, Jean B.},
title = {A disintegration of the {Christoffel} function},
journal = {Comptes Rendus. Math\'ematique},
pages = {1071--1079},
publisher = {Acad\'emie des sciences, Paris},
volume = {360},
number = {G9},
year = {2022},
doi = {10.5802/crmath.380},
language = {en},
url = {http://archive.numdam.org/articles/10.5802/crmath.380/}
}
TY - JOUR AU - Lasserre, Jean B. TI - A disintegration of the Christoffel function JO - Comptes Rendus. Mathématique PY - 2022 SP - 1071 EP - 1079 VL - 360 IS - G9 PB - Académie des sciences, Paris UR - http://archive.numdam.org/articles/10.5802/crmath.380/ DO - 10.5802/crmath.380 LA - en ID - CRMATH_2022__360_G9_1071_0 ER -
%0 Journal Article %A Lasserre, Jean B. %T A disintegration of the Christoffel function %J Comptes Rendus. Mathématique %D 2022 %P 1071-1079 %V 360 %N G9 %I Académie des sciences, Paris %U http://archive.numdam.org/articles/10.5802/crmath.380/ %R 10.5802/crmath.380 %G en %F CRMATH_2022__360_G9_1071_0
Lasserre, Jean B. A disintegration of the Christoffel function. Comptes Rendus. Mathématique, Tome 360 (2022) no. G9, pp. 1071-1079. doi : 10.5802/crmath.380. http://archive.numdam.org/articles/10.5802/crmath.380/
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