Erd\H{o}s-Ko-Rado Theorem for Bounded Multisets

Let $k, m, n$ be positive integers with $k \geq 2$. A $k$-multiset of $[n]_m$ is a collection of $k$ integers from the set $\{1, 2, \ldots, n\}$ in which the integers can appear more than once but at most $m$ times. A family of such $k$-multisets is called an intersecting family if every pair of $k$-multisets from the family have non-empty intersection. A finite sequence of real numbers $\{a_1,a_2,\ldots,a_n\}$ is said to be unimodal if there is some $k\in \{1,2,\ldots,n\}$, such that $a_1\leq a_2\leq\ldots\leq a_{k-1}\leq a_k\geq a_{k+1}\geq \ldots\geq a_n$. Given $m,n,k$, denote $C_{k,l}$ as the coefficient of $x^k$ in the generating function $(\sum_{i=1}^mx^i)^l$, where $1\leq l\leq n$. In this paper, we first show that the sequence of $\{C_{k,1},C_{k,2},\ldots,C_{k,n}\}$ is unimodal. Then we use this as a tool to prove that the intersecting family in which every $k$-multiset contains a fixed element attains the maximum cardinality for $n \geq k + \lceil k/m\rceil$. In the special case when $m = 1$ and $m=\infty$, our result gives rise to the famous Erd\H{o}s-Ko-Rado Theorem and an unbounded multiset version for this problem given by Meagher and Purdy, respectively. The main result in this paper can be viewed as a bounded multiset version of the Erd\H{o}s-Ko-Rado Theorem.