1. Department of Mathematics, J. A. College for Women, Periyakulam, Theni, India
2. Research Group on Mathematical Linguistics, Rovira i Virgili University, PL Imperial, Tarraco 1, 43005 Tarragona, Spain
3. Department of Mathematics, Madras Christian College, Chennai, India
4. Department of Mathematics of Computer Sciences, Institute of Mathematics, NCST, 18 Hoang Quoc Viet Road, 10307, Hanoi, Vietnam
5. College of Computer Science, Northeastern University, 360 Huntington Avenue #221, CN, Boston MA 02115, USA