A078542 Unbalanced composite numbers.

4, 8, 9, 10, 16, 18, 20, 21, 22, 24, 25, 26, 27, 28, 32, 33, 34, 36, 38, 39, 40, 44, 45, 46, 48, 49, 50, 51, 52, 54, 55, 57, 58, 60, 62, 63, 64, 65, 66, 68, 69, 72, 74, 75, 76, 77, 80, 81, 82, 84, 85, 86, 87, 88, 90, 91, 92, 93, 94, 95, 96, 98, 99, 100, 102, 104, 106, 108

Offset: 1

Author

Labos Elemer, Dec 04 2002

Keywords

• nonn

Examples

			46 = 2*23 and sigma(46)/phi(46) = 72/22 is not an integer, so 46 is in the sequence.
		

Links

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

Crossrefs

Cf. A000010, A000203, A020492, A078538, A078539, A078540.

Programs

Mathematica

Do[s=DivisorSigma[1, n]/EulerPhi[n]; If[ !IntegerQ[s]&&!PrimeQ[n], Print[n]], {n, 1, 256}]
Select[Range[150],CompositeQ[#]&&!IntegerQ[DivisorSigma[1,#]/ EulerPhi[ #]]&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jan 19 2020 *)

PARI

lista(nn) = forcomposite(n=1, nn, if (denominator(sigma(n)/eulerphi(n)) != 1, print1(n, ", "))); \\ Michel Marcus, Jul 11 2018