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Search: id:A063719
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| A063719 |
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Numbers n such that usigma(cototient(n)) is a prime. |
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+0
1
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| 4, 6, 8, 24, 28, 32, 384, 448, 496, 508, 512, 98304, 114688, 126976, 130048, 131056, 131072
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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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 (wasserma(AT)spawar.navy.mil), Jul 09 2002
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EXAMPLE
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131072 is in the sequence because A034448(A051953(131072)) = A034448(65536) = 65537, a prime.
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PROGRAM
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(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)))
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CROSSREFS
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Cf. A034448, A051953.
Sequence in context: A083790 A086561 A074125 this_sequence A106366 A019160 A126233
Adjacent sequences: A063716 A063717 A063718 this_sequence A063720 A063721 A063722
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KEYWORD
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nonn
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AUTHOR
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Jason Earls (zevi_35711(AT)yahoo.com), Aug 23 2001
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