Completing partial $k$-star designs

Abstract

A $k$-star is a complete bipartite graph $K_{1,k}$. A partial $k$-star design of order $n$ is a pair $(V,\mathcal{A})$ where $V$ is a set of $n$ vertices and $\mathcal{A}$ is a set of edge-disjoint $k$-stars whose vertex sets are subsets of $V$. If each edge of the complete graph with vertex set $V$ is in some star in $\mathcal{A}$, then $(V,\mathcal{A})$ is a (complete) $k$-star design. We say that $(V,\mathcal{A})$ is completable if there is a $k$-star design $(V,\mathcal{B})$ such that $\mathcal{A} \subseteq \mathcal{B}$. In this paper we determine, for all $k$ and $n$, the minimum number of stars in an uncompletable partial $k$-star design of order $n$.