Randić energy of digraphs

We assume that D is a directed graph with vertex set ⁡()={1,…} and arc set ⁡(). A VDB topological index φ of D is defined as ⁡()= 1 2 ∑ ⁢∈⁡() + ,−, where + and − denote the outdegree and indegree of vertices u and v, respectively, and , is a bivariate symmetric function defined on nonnegative real n...