Some counting problems of ordered cover of finite set
Qiu Jian-xia · Journal of Guangzhou University · 2004
Some counting problems about empty and unempty ordered cover of finite set were studied here. Through addition principle, multiplication principle and principle of inclusion and exclusion,a series of counting formulas about m groups (A_1,A_2,Λ,A_m) empty (unempty) ordered cover of N_n are obtained, which fit different kinds of conditions.