Affiliation: | (1) Dipartimento di Matematica Pura ed Applicata, Università di L'Aquila, Via Vetoio, 67100 L'Aquila, Italy;(2) Institute of Mathematics, Polish Academy of Sciences, Bankowa 14, 40-007 Katowice, Poland;(3) Department of Mathematics, Technical University of Rzeszów, W. Pola 6, 35-959 Rzeszów, Poland |