A067800 Nonprime numbers k such that phi(k) > k/2.

1, 9, 15, 21, 25, 27, 33, 35, 39, 45, 49, 51, 55, 57, 63, 65, 69, 75, 77, 81, 85, 87, 91, 93, 95, 99, 111, 115, 117, 119, 121, 123, 125, 129, 133, 135, 141, 143, 145, 147, 153, 155, 159, 161, 169, 171, 175, 177, 183, 185, 187, 189, 201, 203, 205, 207, 209, 213, 215

Offset: 1

Author

Benoit Cloitre, Feb 07 2002

Keywords

• nonn

Comments

Sequence is similar to A014076(n) giving odd nonprimes. Only 3 terms = 105, 165, 195 are not in the sequence among 59 terms < 210.

Cototient(m) > totient(m) equivalent to 2*phi(m) < m; the missing terms mentioned here seem to form A036798. - Labos Elemer, May 08 2003

The number 9075 is not in this sequence, is in A014076 and is not in A036798, which means that the missing terms mentioned here do not form A036798 (cf. A118700). - R. J. Mathar, Aug 08 2007

Links

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

Crossrefs

Cf. A000010 (phi), A014076, A036798, A051953, A083254, A118700.

Programs

Mathematica

Select[Range[250],!PrimeQ[#]&&EulerPhi[#]>#/2&] (* Harvey P. Dale, Aug 29 2021 *)

PARI

isok(k) = !isprime(k) && eulerphi(k) > k/2; \\ Amiram Eldar, May 08 2025