OFFSET
1,1
COMMENTS
For every prime p, p^p has p+1 divisors. p=2 is a term, but for all odd primes, p+1 is even, so this sequence consists of 2 and the primes of the form 2^j - 1, i.e., 2 and the Mersenne primes (A000668).
EXAMPLE
2^2 has 3 = 3^1 divisors, so 2 is a term.
3^3 has 4 = 2^2 divisors, so 3 is a term.
5^5 has 6 = 2*3 divisors, so 5 is not a term.
CROSSREFS
KEYWORD
nonn,hard
AUTHOR
Jon E. Schoenfield, Jun 01 2021
STATUS
approved