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Abstract
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The Narumi-Katayama index of a graph G, denoted by NK(G), is equal to the product of degrees
of vertices of G. In this paper we investigate its behavior under several binary operations on graphs. We
present explicit formulas for its values for composite graphs in terms of its values for operands and some
auxiliary invariants. We demonstrate applications of our results to several chemically relevant classes of
graphs and show how the Narumi-Katayama index can be used as a measure of graph irregularity.
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