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A078542 Unbalanced composite numbers. 1
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified June 20 06:56 EDT 2021. Contains 345157 sequences. (Running on oeis4.)