Parameterized Complexity of Vertex Splitting to Pathwidth at most 1
Motivated by the planarization of 2-layered straight-line drawings, we consider the problem of modifying a graph such that the resulting graph has pathwidth at most 1. The problem Pathwidth-One Vertex Explosion (POVE) asks whether such a graph can be obtained using at most $k$ vertex explosions, where a vertex explosion replaces a vertex $v$ by deg$(v)$ degree-1 vertices, each incident to exactly one edge that was originally incident to $v$. For POVE, we give an FPT algorithm with running time $O(4^k \cdot m)$ and an $O(k^2)$ kernel, thereby improving over the $O(k^6)$-kernel by Ahmed et al. [GD 22] in a more general setting. Similarly, a vertex split replaces a vertex $v$ by two distinct vertices $v_1$ and $v_2$ and distributes the edges originally incident to $v$ arbitrarily to $v_1$ and $v_2$. Analogously to POVE, we define the problem variant Pathwidth-One Vertex Splitting (POVS) that uses the split operation instead of vertex explosions. Here we obtain a linear kernel and an algorithm with running time $O((6k+12)^k \cdot m)$. This answers an open question by Ahmed et al. [GD22]. Finally, we consider the problem $Π$ Vertex Splitting ($Π$-VS), which generalizes the problem POVS and asks whether a given graph can be turned into a graph of a specific graph class $Π$ using at most $k$ vertex splits. For graph classes $Π$ that can be tested in monadic second-order graph logic (MSO$_2$), we show that the problem $Π$-VS can be expressed as an MSO$_2$ formula, resulting in an FPT algorithm for $Π$-VS parameterized by $k$ if $Π$ additionally has bounded treewidth. We obtain the same result for the problem variant using vertex explosions.