A new width parameter of graphs based on edge cuts: $α$-edge-crossing width

We introduce graph width parameters, called $α$-edge-crossing width and edge-crossing width. These are defined in terms of the number of edges crossing a bag of a tree-cut decomposition. They are motivated by edge-cut width, recently introduced by Brand et al. (WG 2022). We show that edge-crossing width is equivalent to the known parameter tree-partition-width. On the other hand, $α$-edge-crossing width is a new parameter; tree-cut width and $α$-edge-crossing width are incomparable, and they both lie between tree-partition-width and edge-cut width. We provide an algorithm that, for a given $n$-vertex graph $G$ and integers $k$ and $α$, in time $2^{O((α+k)\log (α+k))}n^2$ either outputs a tree-cut decomposition certifying that the $α$-edge-crossing width of $G$ is at most $2α^2+5k$ or confirms that the $α$-edge-crossing width of $G$ is more than $k$. As applications, for every fixed $α$, we obtain FPT algorithms for the List Coloring and Precoloring Extension problems parameterized by $α$-edge-crossing width. They were known to be W[1]-hard parameterized by tree-partition-width, and FPT parameterized by edge-cut width, and we close the complexity gap between these two parameters.