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Let [Formula: see text] be a simple connected graph. The inverse sum indeg index of [Formula: see text], denoted by [Formula: see text], is defined as the sum of the weights [Formula: see text] of all edges [Formula: see text] of [Formula: see text], where [Formula: see text] denotes the degree of a vertex in [Formula: see text]. In this paper, we derive some bounds for the inverse sum indeg index in terms of some graph parameters, such as vertex (edge) connectivity, chromatic number, vertex bipartiteness, etc. The corresponding extremal graphs are characterized, respectively.

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.

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