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%I #19 Aug 23 2024 08:42:09
%S 4,8,9,10,16,18,20,21,22,24,25,26,27,28,32,33,34,36,38,39,40,44,45,46,
%T 48,49,50,51,52,54,55,57,58,60,62,63,64,65,66,68,69,72,74,75,76,77,80,
%U 81,82,84,85,86,87,88,90,91,92,93,94,95,96,98,99,100,102,104,106,108
%N Unbalanced composite numbers.
%H Amiram Eldar, <a href="/A078542/b078542.txt">Table of n, a(n) for n = 1..10000</a>
%e 46 = 2*23 and sigma(46)/phi(46) = 72/22 is not an integer, so 46 is in the sequence.
%t Do[s=DivisorSigma[1, n]/EulerPhi[n]; If[ !IntegerQ[s]&&!PrimeQ[n], Print[n]], {n, 1, 256}]
%t Select[Range[150],CompositeQ[#]&&!IntegerQ[DivisorSigma[1,#]/ EulerPhi[ #]]&] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Jan 19 2020 *)
%o (PARI) lista(nn) = forcomposite(n=1, nn, if (denominator(sigma(n)/eulerphi(n)) != 1, print1(n, ", "))); \\ _Michel Marcus_, Jul 11 2018
%Y Cf. A000010, A000203, A020492, A078538, A078539, A078540.
%K nonn
%O 1,1
%A _Labos Elemer_, Dec 04 2002