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

1,1

Links

Table of n, a(n) for n=1..68.

Example

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

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 *)

Prog

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

Crossrefs

Cf. A020492, A078538, A078539, A078540.

Sequence in context: A243176 A094906 A312829 * A124982 A342106 A251606

Adjacent sequences: A078539 A078540 A078541 * A078543 A078544 A078545

Keyword

nonn

Author

Labos Elemer, Dec 04 2002

Status

approved