Group With Maximum Undirected Edges in Directed Power Graph Among All   Finite Non-Cyclic Nilpotent Groups
  In [Curtin and Pourgholi, A group sum inequality and its application to power graphs, J. Algebraic Combinatorics, 2014], it is proved that among all directed power graphs of groups of a given order $ n $, the directed power graph of cyclic group of order $ n $ has the maximum number of undirected edges. In this paper, we continue their work and we determine a non-cyclic nilpotent group of an odd order $ n $ whose directed power graph has the maximum number of undirected edges among all non-cyclic nilpotent groups of order $n$.   We next determine non-cyclic $p$-groups whose undirected power graphs have the maximum number of edges among all groups of the same order. 
P. Darbari and B. Khosravi
math.CO