Institution: | a Université du Luxembourg, Campus Kirchberg, 6 Rue Coudenhove-Kalergi, L-1359 Luxembourg b Institute of Mathematics and Informatics, University of Opole, 45-052 Opole, Poland c Dpto de Análisis Matemático, Universidad de Valladolid, Facultad de Ciencias, 47011 Valladolid, Spain |