Extremal phenylene chains with respect to the Mostar index

Abstract

For a connected graph [Formula: see text], the Mostar index is defined as [Formula: see text], where [Formula: see text] (respectively, [Formula: see text]) is the number of vertices of [Formula: see text] closer to [Formula: see text] (respectively, [Formula: see text]) than [Formula: see text] (respectively, [Formula: see text]). In this paper, we determine the first five maximal (respectively, the first five minimal) values of the Mostar index among all phenylene chains with [Formula: see text] hexagons, the corresponding extremal chains are completely characterized, respectively.