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A078892 Numbers n such that phi(n) - 1 is prime, where phi is Euler's totient function (A000010). 8
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

MATHEMATICA

Select[Range[200], PrimeQ[EulerPhi[#] - 1]&] (* Vincenzo Librandi, Aug 13 2013 *)

PROG

(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

CROSSREFS

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

KEYWORD

nonn,easy

AUTHOR

Reinhard Zumkeller, Dec 12 2002

STATUS

approved

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Last modified December 7 18:12 EST 2019. Contains 329847 sequences. (Running on oeis4.)