Some results on concatenating bipartite graphs

Abstract

We consider two functions $\ddot{I}•$ and $\ddot{I}ˆ$, defined as follows. Let $x,y\in (0,1]$ and let $A,B,C$ be disjoint nonempty subsets of a graph $G$, where every vertex in $A$ has at least $x|B|$ neighbors in $B$, and every vertex in $B$ has at least $y|C|$ neighbors in $C$. We denote by $\ddot{I}•(x,y)$ the maximum $z$ such that, in all such graphs $G$, there is a vertex $v\in C$ that is joined to at least $z|A|$ vertices in $A$ by two-edge paths. If in addition we require that every vertex in $B$ has at least $x|A|$ neighbors in $A$, and every vertex in $C$ has at least $y|B|$ neighbors in $C$, we denote by $\ddot{I}ˆ(x,y)$ the maximum $z$ such that, in all such graphs $G$, there is a vertex $v\in C$ that is joined to at least $z|A|$ vertices in $A$ by two-edge paths. In their recent paper, M. Chudnovsky, P. Hompe, A. Scott, P. Seymour, and S. Spirkl introduced these functions, proved some general results about them, and analyzed when they are greater than or equal to $1/2,2/3,$ and $1/3$. Here, we extend their results by analyzing when they are greater than or equal to $3/4,2/5,$ and $3/5$.

Results of paper edited and incorporated into arXiv:1902.10878