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 A039698 Numbers n such that phi(n) + 1 is prime. 12
 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 17, 18, 19, 21, 22, 23, 26, 27, 28, 29, 31, 32, 34, 36, 37, 38, 40, 41, 42, 43, 46, 47, 48, 49, 53, 54, 55, 57, 58, 59, 60, 61, 62, 63, 67, 71, 73, 74, 75, 76, 77, 79, 82, 83, 86, 88, 89, 91, 93, 94, 95, 97, 98, 99, 100, 101, 103 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Positive integers n for which values of A039649(n) are primes. - Vladimir Shevelev, May 10 2008 For every prime p, the numbers p and 2p are terms of this sequence. - Vladimir Shevelev, May 10 2008 Union of A000040 and A066071. - Ray Chandler, May 26 2008 LINKS Antti Karttunen, Table of n, a(n) for n = 1..26197 (first 1000 terms from Vincenzo Librandi) EXAMPLE phi(10)+1=4+1=5, a prime number. MATHEMATICA Select[Range[300], PrimeQ[EulerPhi[#] + 1]&] (* Vincenzo Librandi, Aug 13 2013 *) PROG (MAGMA) [n: n in [1..200] | IsPrime(EulerPhi(n)+1)]; // Vincenzo Librandi, Aug 13 2013 CROSSREFS Cf. A000010, A000040, A006093, A039649, A066071, A007614. Cf. A039689 (complement), A296079 (characteristic function). Cf. also A065512, A078892, A263028, A248792. Sequence in context: A166535 A247764 A302140 * A078107 A072089 A072088 Adjacent sequences:  A039695 A039696 A039697 * A039699 A039700 A039701 KEYWORD nonn,easy AUTHOR EXTENSIONS Edited by N. J. A. Sloane, May 21 2008 at the suggestion of R. J. Mathar STATUS approved

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Last modified June 14 21:27 EDT 2021. Contains 345041 sequences. (Running on oeis4.)