1. Departamento de Matemática; Instituto de Ciências Exatas, Universidade Federal do Amazonas; 69060-900 Manaus Brazil
2. Department of Mathematics; King Khalid University; Guraiger Abha 62529 Saudi Arabia
3. Department of Mathematics and Computer Science Victoriei 76; North University Center of Baia Mare, Technical University of Cluj Napoca; 430122 Baia Mare Romania