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 ; and Research group of the project PN-III-P4-ID-PCE-2016-0035, “Simion Stoilow” Institute of Mathematics of the Romanian Academy, 010702 Bucharest, Romania