OFFSET
1,1
COMMENTS
All these composite numbers k should be prime powers because if k=a*b with gcd(a,b)=1, then sigma(aabb) = sigma(aa)*sigma(bb) cannot be a prime; 46 of the 236 prime powers below 1000000 are here.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..500 (terms 1..70 from Harry J. Smith)
MATHEMATICA
Select[ Range[3 10^5], ! PrimeQ[ # ] && PrimeQ[ DivisorSigma[1, #^2]] & ]
PROG
(PARI) { n=0; for (m=1, 10^9, if (isprime(m), next); x=sigma(m^2); if (isprime(x), write("b065405.txt", n++, " ", m); if (n==70, return)) ) } \\ Harry J. Smith, Oct 18 2009
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Nov 06 2001
STATUS
approved