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A066071
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Nonprime numbers n such that phi(n) + 1 is prime.
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13
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1, 4, 6, 8, 9, 10, 12, 14, 18, 21, 22, 26, 27, 28, 32, 34, 36, 38, 40, 42, 46, 48, 49, 54, 55, 57, 58, 60, 62, 63, 74, 75, 76, 77, 82, 86, 88, 91, 93, 94, 95, 98, 99, 100, 106, 108, 110, 111, 114, 115, 117, 118, 119, 122, 124, 125, 126, 132, 133, 134, 135, 142, 145, 146
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OFFSET
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1,2
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COMMENTS
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A039698 with the primes removed. For every prime p, 2p is in the sequence. - Ray Chandler, May 26 2008
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LINKS
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EXAMPLE
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Solutions to 1+phi(x)=13 are {13, 21, 26, 28, 36, 42} of which the 5 composites are in the sequence.
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MAPLE
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select(n -> not isprime(n) and isprime(1+numtheory:-phi(n)), [$1..1000]); # Robert Israel, Dec 29 2017
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MATHEMATICA
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Select[Complement[Range@ #, Prime@ Range@ PrimePi@ #] &@ 150, PrimeQ[EulerPhi@ # + 1] &] (* Michael De Vlieger, Jul 01 2016 *)
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PROG
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(PARI) { n=0; for (m=1, 10^9, if (!isprime(m) && isprime(eulerphi(m) + 1), write("b066071.txt", n++, " ", m); if (n==1000, return)) ) } \\ Harry J. Smith, Nov 10 2009
(Magma) [n: n in [1..200] |not IsPrime(n) and IsPrime(EulerPhi(n)+1)]; // Vincenzo Librandi, Jul 02 2016
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CROSSREFS
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Cf. A000010, A000040, A005382, A006093, A039649, A039698, A058339, A058340, A065508, A066072, A066073, A066074, A066075, A066076, A066077, A066080.
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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