Efficient Isomorphism for $S_d$-graphs and $T$-graphs

An $H$-graph is one representable as the intersection graph of connected subgraphs of a suitable subdivision of a fixed graph $H$, introduced by Biró, Hujter and Tuza (1992). An $H$-graph is proper if the representing subgraphs of $H$ can be chosen incomparable by the inclusion. In this paper, we focus on the isomorphism problem for $S_d$-graphs and $T$-graphs, where $S_d$ is the star with $d$ rays and $T$ is an arbitrary fixed tree. Answering an open problem of Chaplick, Töpfer, Voborník and Zeman (2016), we provide an FPT-time algorithm for testing isomorphism and computing the automorphism group of $S_d$-graphs when parameterized by~$d$, which involves the classical group-computing machinery by Furst, Hopcroft, and Luks (1980). We also show that the isomorphism problem of $S_d$-graphs is at least as hard as the isomorphism problem of posets of bounded width, for which no efficient combinatorial-only algorithm is known to date. Then we extend our approach to an XP-time algorithm for isomorphism of $T$-graphs when parameterized by the size of $T$. Lastly, we contribute a simple FPT-time combinatorial algorithm for isomorphism testing in the special case of proper $S_d$- and $T$-graphs.