1. Department of Applied Mathematics, National University of Mongolia, University Street 3, Ulaanbaatar 21-046, Mongolia
2. Department of Mathematical Sciences, Aalborg University, Skjernvej 4A, DK-9220 Aalborg Ø, Denmark
3. Department of Mathematics, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan