The ABC (atom-bond connectivity) matrix of a simple graph is a weighted adjacency matrix with weight
(1)
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where are the vertex degrees of the graph. In other words,
(2)
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(Zheng et al. 2022).
It was introduced by Estrada et al. (2017) to model the enthalpy of formation of alkanes (Zheng et al. 2023).
Its largest eigenvalue is called the ABC spectral radius, half the sum of its matrix elements is the ABC index, and the sum of absolute values of its eigenvalues is the ABC energy.