“…There is a trend in graph theory of studying discrete versions of concepts that classically appear in differential geometry (see for example [1], [2], [3], [5], [6], [7], [10], [9], [11], [13] , [16], [20], [22], [23], [24], [25]). In particular, a definition of Ricci curvature for graphs was given in 1999 by Schmuckenschläger.…”