1. Department of Mathematics, University of Nicosia, P.O. Box 24005, 1700 Nicosia, Cyprus.
2. School of Mathematics, Statistics and Computer Science, University of KwaZulu-Natal, Private Bag X54001 Durban 4000, Republic of South Africa
3. Institute of Systems Science and Department of Mathematics, Durban University of Technology, P.O. Box 1433, Durban 4000, Republic of South Africa.
4. Department of Mathematics, SASTRA Deemed University, Thanjavur, India 613401.