The Randić index, also known as the connectivity index, of a graph is the sum of bond contributions 1 / ( d i d j ) 1 / 2 {\displaystyle 1/(d_{i}d_{j})^{1/2}} where d i {\displaystyle d_{i}} and d j {\displaystyle d_{j}} are the degrees of the vertices making bond i ~ j.
History This graph invariant was introduced by Milan Randić in 1975. It is often used in chemoinformatics for investigations of organic compounds.
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References Roberto Todeschini, Viviana Consonni (2009) "Molecular Descriptors for Chemoinformatics", Wiley-VCH, ISBN 978-3-527-31852-0 Li, Xueliang; Shi, Yongtang (2008), "A survey on the Randić index", MATCH Communications in Mathematical and in Computer Chemistry, 59 (1): 127–156.