Since many real-world graphs are nonplanar, the study of graphs that allow few crossings per edge has been an active subfield of graph theory in recent years. One of the most natural generalizations of planar graphs are the so-called 1-planar graphs that admit a drawing with at most one crossing per edge. Unfortunately, testing whether a graph is 1-planar is known to be NP-complete even for very restricted graph classes. On the positive side, Binucci, Didimo and Montecchiani [Binucci et al., 2023] presented the first practical algorithm for testing 1-planarity based on an easy-to-implement backtracking strategy. We build on this idea and systematically explore the design choices of such algorithms and propose several new ingredients, such as different branching strategies and multiple filter criteria that allow us to reject certain branches in the search tree early on. We conduct an extensive experimental evaluation that evaluates the efficiency and effectiveness of these ingredients. Given a time limit of three hours per instance, our best configuration is able to solve more than 95% of the non-planar instances from the well-known North and Rome graphs with up to 50 vertices. Notably, the median running time for solved instances is well below 4 seconds.

We consider three simple quadratic time algorithms for the problem Level Planarity and give a level-planar instance that they either falsely report as negative or for which they output a drawing that is not level planar.

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(4𝑘 ⋅ 𝑚) and an 𝑂(𝑘2) kernel, thereby improving over the 𝑂(𝑘6) kernel by Ahmed et al. [2] in a more general setting. Similarly, a vertex split replaces a vertex 𝑣 by two distinct vertices 𝑣1 and 𝑣2 and distributes the edges originally incident to 𝑣 arbitrarily to 𝑣1 and 𝑣2. 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 ⋅ 𝑚). This answers an open question by Ahmed et al. [2]. Finally, we consider the problem Π-VertexSplitting (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 dfined in monadic second-order graph logic (MSO2), we show that the problem Π-VS can be expressed as an MSO2 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. [2] R. Ahmed, S.G. Kobourov, M. Kryven, An FPT algorithm for bipartite vertex splitting, in: P. Angelini, R. von Hanxleden (Eds.), Graph Drawing and Network Visualization -30th International Symposium, GD 2022, in: Lecture Notes in Computer Science, vol.13764, Springer, 2022, pp.261--268.