Proving the pressing game conjecture on linear graphs

Abstract

The pressing game on black-and-white graphs is the following: given a graph $G\left(V,E\right)$ with its vertices colored with black and white, any black vertex $v$ can be pressed, which has the following effect: (1) all neighbors of $v$ change color; i.e., white neighbors become black and vice versa; (2) all pairs of neighbors of $v$ change adjacency; i.e., adjacent pairs become nonadjacent and nonadjacent ones become adjacent; and (3) $v$ becomes a separated white vertex. The aim of the game is to transform $G$ into an all-white, empty graph. It is a known result that the all-white empty graph is reachable in the pressing game if each component of $G$ contains at least one black vertex, and for a fixed graph, any successful transformation has the same number of pressed vertices.

The pressing game conjecture states that any successful pressing sequence can be transformed into any other successful pressing sequence with small alterations. Here we prove the conjecture for linear graphs, also known as paths. The connection to genome rearrangement and sorting signed permutations with reversals is also discussed.