1. Department of Mathematics and Statistics, Loyola University Chicago, 1032 W. Sheridan Road, Chicago, IL 60660, USA
2. Department of Mathematics, University of Craiova, 200585 Craiova, Romania
3. Research Group of the Project PN-III-P4-ID-PCE-2016-0035, “Simion Stoilow” Institute of Mathematics of the Romanian Academy, 010702 Bucharest, Romania