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 A063719 Numbers n such that usigma(cototient(n)) is a prime. 0
 4, 6, 8, 24, 28, 32, 384, 448, 496, 508, 512, 98304, 114688, 126976, 130048, 131056, 131072 (list; graph; refs; listen; history; text; internal format)
 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 LINKS EXAMPLE 131072 is in the sequence because A034448(A051953(131072)) = A034448(65536) = 65537, a prime. 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 Cf. A034448, A051953. Sequence in context: A083790 A217201 A074125 * A272862 A361662 A106366 Adjacent sequences: A063716 A063717 A063718 * A063720 A063721 A063722 KEYWORD nonn AUTHOR Jason Earls, Aug 23 2001 STATUS approved

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Last modified March 25 04:11 EDT 2023. Contains 361511 sequences. (Running on oeis4.)