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

Author

Reinhard Zumkeller, Dec 12 2002

Keywords

• nonn

Comments

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

Links

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

Crossrefs

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

Programs

Mathematica

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

PARI

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