Group invariant separating polynomials on a Banach space

  1. Falcó, Javier 1
  2. García, Domingo 1
  3. Maestre, Manuel 1
  4. Jung, Mingu 2
  1. 1 Universidad de Valencia. Departamento de Análisis Matemático
  2. 2 POSTECH. Department of Mathematics.
Revista:
Publicacions matematiques

ISSN: 0214-1493

Año de publicación: 2022

Volumen: 66

Número: 1

Páginas: 207-233

Tipo: Artículo

Otras publicaciones en: Publicacions matematiques

Resumen

We study the group-invariant continuous polynomials on a Banach space X that separate a given set K in X and a point z outside K. We show that if X is a real Banach space, G is a compact group of L(X), K is a G-invariant set in X, and z is a point outside K that can be separated from K by a continuous polynomial Q, then z can also be separated from K by a G-invariant continuous polynomial P. It turns out that this result does not hold when X is a complex Banach space, so we present some additional conditions to get analogous results for the complex case. We also obtain separation theorems under the assumption that X has a Schauder basis which give applications to several classical groups. In this case, we obtain characterizations of points which can be separated by a group-invariant polynomial from the closed unit ball.

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