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%I #9 Dec 15 2017 17:35:23
%S 4,6,8,24,28,32,384,448,496,508,512,98304,114688,126976,130048,131056,
%T 131072
%N Numbers n such that usigma(cototient(n)) is a prime.
%C 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
%e 131072 is in the sequence because A034448(A051953(131072)) = A034448(65536) = 65537, a prime.
%o (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)))
%Y Cf. A034448, A051953.
%K nonn
%O 1,1
%A _Jason Earls_, Aug 23 2001
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