Brenton, L. and Drucker, D. "Perfect Graphs and Complex Surface Singularities with Perfect Local Fundamental Group." Tôhoku Math. J.41, 507-525, 1989.Butske, W.; Jaje, L. M.; and Mayernik, D. R. "The Equation , Pseudoperfect Numbers, and Partially Weighted Graphs." Math. Comput.69, 407-420, 1999.