Distance-regular graphs having the M-property

We analyse when the Moore-Penrose inverse of the combinatorial Laplacian of a distance-regular graph is an M-matrix; that is, it has non-positive off-diagonal elements or, equivalently when the Moore-Penrose inverse of the combinatorial Laplacian of a distance-regular graph is also the combinatorial Laplacian of another network. When this occurs we say that the distance-regular graph has the M-property. We prove that only distance-regular graphs with diameter up to three can have the M-property and we give a characterization of the graphs that satisfy the M-property in terms of their intersection array. Moreover, we exhaustively analyse strongly regular graphs having the M-property and we give some families of distance-regular graphs with diameter three that satisfy the M-property. Roughly speaking, we prove that all distance-regular graphs with diameter one; about half of the strongly regular graphs; only some imprimitive distance-regular graphs with diameter three, and no distance-regular graphs with diameter greater than three, have the M-property. In addition, we conjecture that no primitive distance-regular graph with diameter three has the M-property.