[1] Univ Elect Sci & Technol China, Sch Math Sci, Inst Computat Sci, Chengdu 611731, Sichuan, Peoples R China
[2] Univ Groningen, Johann Bernoulli Inst Math & Comp Sci, Nijenborgh 9,POB 407, NL-9747 AG Groningen, Netherlands
[3] Free Univ Bozen Bolzano, Fac Comp Sci, Piazza Domenicani 3, I-39100 Bolzano, Italy