1. Deparment of Mathematics, K.K. Education Faculty, Atatürk University, 25240 Erzurum, Turkey
2. Facultad de Ciencias Exactas y Naturales y Agrimensura, Universidad Nacional del Nordeste, Av. Libertad 5450, Corrientes 3400, Argentina
3. Department of Mathematics, Technical University of Cluj-Napoca, 400020 Cluj-Napoca, Romania