OFFSET
1,1
COMMENTS
If usigma(x) is prime, it must be a Fermat prime. It is conjectured that there are only 5 Fermat primes. If this conjecture is true, this sequence has no more terms. - David Wasserman, Jul 09 2002
EXAMPLE
PROG
(PARI) u(n) = sumdiv(n, d, if(gcd(d, n/d)==1, d)); c(n) = n-eulerphi(n); for(n=1, 10^8, if(isprime(u(c(n))), print(n)))
CROSSREFS
KEYWORD
nonn
AUTHOR
Jason Earls, Aug 23 2001
STATUS
approved