A078893 Composite numbers k such that phi(k) - 1 is prime, where phi is Euler's totient function (A000010).

8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 24, 25, 26, 27, 28, 30, 33, 35, 36, 38, 39, 42, 44, 45, 49, 50, 51, 52, 54, 56, 62, 64, 65, 66, 68, 69, 70, 72, 77, 78, 80, 81, 84, 86, 90, 91, 92, 93, 95, 96, 98, 99, 102, 104, 105, 111, 112, 117, 120, 121, 122, 123, 124, 129, 130, 133

Offset

1,1

Comments

A078892 with the primes removed. - Ray Chandler, May 26 2008

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Mathematica

Select[Range[150], CompositeQ[#]&&PrimeQ[EulerPhi[#]-1]&] (* Harvey P. Dale, Dec 28 2021 *)

Prog

(PARI) is(n)=!isprime(n) && isprime(eulerphi(n)-1) \\ Charles R Greathouse IV, Feb 21 2013

Crossrefs

Cf. A000010, A000040, A008864, A109606, A078892, A066071, A068019.

Sequence in context: A097363 A152870 A181723 * A050704 A024884 A031009

Adjacent sequences: A078890 A078891 A078892 * A078894 A078895 A078896

Keyword

nonn

Author

Reinhard Zumkeller, Dec 12 2002

Status

approved