Perfect divisor of n is divisor d such that d^k = n for some k >= 1. See A175067 (sum of perfect divisors of n) and A175081 (values taken by the sum of perfect divisors of n (A175067) sorted into ascending order).

Complement of A001597(n+1) for n >= 1 (perfect powers >= 4). a(n) = A007916(n-1) for n >= 2. [From Jaroslav Krizek, Jan 30 2010]