OFFSET
1,2
COMMENTS
sigma(x) cannot be prime unless x is a square of a prime power, x = p^2m, cf. A055638 and A023194. This sequence lists the complement: prime powers whose square does not have a prime sum of divisors.
Although generally 1 is not considered a prime power, it seemed logical for various good reasons to include the initial term a(1)=1.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
PROG
(PARI) for(n=1, 999, isprimepower(n)||next; isprime(sigma(n^2))||print1((n)", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
M. F. Hasler, Oct 18 2014
STATUS
approved