Generalized Sharp Bounds on the Spectral Radius of Digraphs
  The spectral radius {\rho}(G) of a digraph G is the maximum modulus of the eigenvalues of its adjacency matrix. We present bounds on {\rho}(G) that are often tighter and are applicable to a larger class of digraphs than previously reported bounds. Calculating the final bound pair is particularly suited to sparse digraphs.   For strongly connected digraphs, we derive equality conditions for the bounds, relating to the outdegree regularity of the digraph. We also prove that the bounds hold with equality only if {\rho}(G) is the r-th root of an integer, where r divides the index of imprimitivity of G. 
Brian K. Butler, Paul H. Siegel
math.CO math.RA