(1) Fachbereich Mathematik, Technische Hochschule Darmstadt, Schlossgartenstrasse 7, 64289 Darmstadt, Germany;(2) Departamento de Matemática, Instituto Superior Técnico, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal
Abstract:
Dedicated to Professor Erhard Meister on the occasion of his 65th birthday.