The Degree-Product Index of Narumi and Katayama
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Let G = (V, E) be a simple graph with vertex set V and edge set E (|V| = n, E = m), and let di be the degree of vertex i of G. The degree product P(G) :=ni=1di of G was introduced and studied by Narumi and Katayama. This index is here fundamentally characterized; first, as the number of "functional" subgraphs of the directed graph D(G) associated to G; and, second, as a suitable weighting over a certain class of ordinary sub-graph covers of G. Then, P(G) is related to several other common graph invariants by way of several bounding relations.