Pure pairs. IV. Trees in bipartite graphs

In this paper we investigate the bipartite analogue of the strong Erdos-Hajnal property. We prove that for every forest $H$ and every $\tau>0$ there exists $\epsilon>0$, such that if $G$ has a bipartition $(A,B)$ and does not contain $H$ as an induced subgraph, and has at most $(1-\tau)|A|\cdot|B|$ edges, then there is a stable set in $G$ that contains at least $\epsilon|V_i|$ vertices of $V_i$, for $i=1,2$. No graphs $H$ except forests have this property.