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A078892
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Numbers n such that phi(n) - 1 is prime, where phi is Euler's totient function (A000010).
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8
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5, 7, 8, 9, 10, 12, 13, 14, 15, 16, 18, 19, 20, 21, 24, 25, 26, 27, 28, 30, 31, 33, 35, 36, 38, 39, 42, 43, 44, 45, 49, 50, 51, 52, 54, 56, 61, 62, 64, 65, 66, 68, 69, 70, 72, 73, 77, 78, 80, 81, 84, 86, 90, 91, 92, 93, 95, 96, 98, 99, 102, 103, 104, 105, 109, 111, 112, 117
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history;
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OFFSET
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1,1
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COMMENTS
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For all primes p: p is in the sequence iff p is the greater member of a twin prime pair (A006512), see A078893.
Union of A006512 and A078893. - Ray Chandler, May 26 2008
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LINKS
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Vincenzo Librandi, Table of n, a(n) for n = 1..1000
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MATHEMATICA
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Select[Range[200], PrimeQ[EulerPhi[#] - 1]&] (* Vincenzo Librandi, Aug 13 2013 *)
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PROG
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(PARI) is(n)=isprime(eulerphi(n)-1) \\ Charles R Greathouse IV, Feb 21 2013
(MAGMA) [n: n in [1..200] | IsPrime(EulerPhi(n)-1)]; // Vincenzo Librandi, Aug 13 2013
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CROSSREFS
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Cf. A000010, A000040, A008864, A109606, A006512, A039698, A072281, A078893.
Sequence in context: A043602 A243624 A038609 * A164374 A072281 A242273
Adjacent sequences: A078889 A078890 A078891 * A078893 A078894 A078895
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KEYWORD
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nonn,easy
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AUTHOR
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Reinhard Zumkeller, Dec 12 2002
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STATUS
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approved
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