Non-existence of bipartite graphs of diameter at least 4 and defect 2

The Moore bipartite bound represents an upper bound on the order of a bipartite graph of maximum degree Δ and diameter D. Bipartite graphs of maximum degree Δ, diameter D and order equal to the Moore bipartite bound are called Moore bipartite graphs. Such bipartite graphs exist only if D=2,3,4 and 6, and for D=3,4,6, they have been constructed only for those values of Δ such that Δ−1 is a prime power.