An analogue of the Erdos-Ko-Rado theorem for weak compositions

摘要

Let n0 be the set of non-negative integers, and let P(n,l) denote the set of all weak compositions of n with l parts, i.e., P(n,l)={(x _1,x_2,?,x_1)∈?_0 ~l:x_1+x_2+,?+x_1=n}. For any element u=(u_1,u_2?,u_l)∈P(n,l), denote its ith-coordinate by u(i), i.e., u(i)=~(ui). A family ?P(n,l) is said to be t-intersecting if |{i:u(i)=v(i)}|≥t for all u,v∈A. We prove that given any positive integers l,t with l≥t+2, there exists a constant n_0(l,t) depending only on l and t, such that for all n≥n_0(l,t), if ?P(n,l) is t-intersecting then |A|≤(n+l-t-1 l-t-1). Moreover, the equality holds if and only if A={u∈P(n,l):u(j)=0for allj∈T} for some t-set T of {1,2,?,l}.