On the bend number of circular-arc graphs as edge intersection graphs of paths on a grid.

LILIANA ALCÓN; FLAVIA BONOMO; GUILLERMO DURÁN; MARISA GUTIERREZ; MARÍA PÍA MAZZOLENI; BERNARD RIES; MARIO VALENCIA-PABON

Haifa

Workshop; 15 th Haifa Workshop on Interdisciplinary Applications of Graphs, Combinatorics and Algorithms; 2015

Golumbic, Lipshteyn and Stern proved that every graph can be represented as the edge intersection graph of paths on a grid (EPG graph), i.e., one can associate with each vertex of the graph a nontrivial path on a rectangular grid such that two vertices are adjacent if and only if the corresponding paths share at least one edge of the grid. For a nonnegative integer k, Bk-EPG graphs are defined as EPG graphs admitting a model in which each path hasat most k bends. Circular-arc graphs are intersection graphs of open arcs of a circle. It is easy to see that every circular-arc graph is a B4-EPG graph, by embedding the circle into a rectangle of the grid. In this paper, we prove that circular-arc graphs are B3-EPG, and that there exist circular-arc graphs which are not B2-EPG. If we restrict ourselves to rectangular representations (i.e., the union of the paths used in the model is contained in the boundaryof a rectangle of the grid), we obtain EPR (edge intersection of paths in a rectangle) representations. We may define Bk-EPR graphs, k>= 0, the same way as Bk-EPG graphs. Circular-arc graphs are clearly B4-EPR graphs and we will show that there exist circular-arc graphs that are not B3-EPR graphs. We also show that normal circular-arc graphs are B2-EPR graphs and that there exist normal circular-arc graphs that are not B1-EPR graphs. Finally, we characterize B1-EPR graphs by a family of minimal forbidden inducedsubgraphs, and show that they form a subclass of normal Helly circular-arc graphs.