1. SCHOOL OF MATHEMATICS AND STATISTICS; YUNNAN UNIVERSITY; KUNMING YUNNAN 650091 P. R. CHINA
2. SCHOOL OF MATHEMATICS AND STATISTICS; THE UNIVERSITY OF WESTERN AUSTRALIA; CRAWLEY WA 6009 AUSTRALIA
3. CENTER FOR COMBINATORICS; LPMC-TJKLC, NANKAI UNIVERSITY; TIANJIN 300071 P. R. CHINA
4. DEPARTMENT OF APPLIED MATHEMATICS; ANHUI UNIVERSITY OF TECHNOLOGY; MAANSHAN 243002 P. R. CHINA