On Efficient Domination for Some Classes of $H$-Free Bipartite Graphs

A vertex set $D$ in a finite undirected graph $G$ is an {\em efficient dominating set} (\emph{e.d.s.}\ for short) of $G$ if every vertex of $G$ is dominated by exactly one vertex of $D$. The \emph{Efficient Domination} (ED) problem, which asks for the existence of an e.d.s.\ in $G$, is known to be \NP-complete even for very restricted $H$-free graph classes such as for $2P3$-free chordal graphs while it is solvable in polynomial time for $P6$-free graphs. Here we focus on $H$-free bipartite graphs: We show that (weighted) ED can be solved in polynomial time for $H$-free bipartite graphs when $H$ is $P7$ or $\ell P4$ for fixed $\ell$, and similarly for $P9$-free bipartite graphs with vertex degree at most 3, and when $H$ is $S{2,2,4}$. Moreover, we show that ED is \NP-complete for bipartite graphs with diameter at most 6.