Affiliation: | 1.Institut de Mathématiques et de Sciences Physiques,Dangbo,Benin;2.School of Mathematics,University of the Witwatersrand,Wits,South Africa;3.Department of Mathematics, Faculty of Sciences,University of Ostrava,Ostrava 1,Czech Republic;4.Department of Mathematics, Statistics and Computer Science,Purdue University Northwest,Westville,USA |