Institution: | 1. Department of Mathematics and Computer Science , Hanover College , Hanover, Indiana, USA katsov@hanover.edu;3. Department of Mathematics , Dong Thap Pedagogical University , Cao Lanh City, Dong Thap, Vietnam , Department of Mathematics, Vinh University, Vinh City, Nghe An, Vietnam;4. Department of Mathematics (DHSP) , Hue University , Hue, Vietnam |