Packing Designs with large block size

Abstract: Given positive integers $v,k,t$ and $\lambda$ with $v \geq k \geq t$, a {packing design} PD${\lambda}(t,k,v)$ is a pair $(V,\mathcal{B})$, where $V$ is a $v$-set and $\mathcal{B}$ is a collection of $k$-subsets of $V$ such that each $t$-subset of $V$ appears in at most $\lambda$ elements of $\mathcal{B}$. The maximum size of a PD${\lambda}(t,k,v)$ is called the {packing number}, denoted PDN${\lambda}(t,k,v)$. In this paper we consider packing designs with $t=2$ and $k$ large (linear) relative to $v$. We provide a new bound on the packing number: if $n \geq \lambda+2$ and $nk-\binom{n}{\lambda+1} \leq \lambda v < (n+1)k-\binom{n+1}{\lambda+1}$, then PDN${\lambda}(2,k,v) \leq n$. When $\lambda=1$ this is equivalent to the second Johnson bound. We show that this bound is met when $\lambda=1$ and $n$ is a positive integer, and when $\lambda=2$ and $2 \leq n \leq 6$. We also consider a directed variant and show that for the cases we consider, this has the same packing number as the undirected case with $\lambda=2$.