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%I #12 Aug 13 2019 11:22:58
%S 15,85,259,295,391,589,799,1111,1717,3193,4171,4369,12361,17473,23533,
%T 25429,28243,31351,34477,36181,41407,47989,51143,52537,58103,65641,
%U 68377,69541,69919,70453,72619,81121,83131,83767,85069,91759
%N Numbers n such that sigma_3(n) is divisible by square of cototient of n, while n is not a prime number.
%H Amiram Eldar, <a href="/A091286/b091286.txt">Table of n, a(n) for n = 1..1000</a>
%e n=15: cototient(15) = 7, sigma_3(15) = 3528 = 72 * 49.
%t Do[s=DivisorSigma[3, n]/(n-EulerPhi[n])^2; If[IntegerQ[s]&&!PrimeQ[n], Print[n]], {n, 1, 100000}]
%o (PARI) isok(n) = (n!=1) && !isprime(n) && !(sigma(n, 3)%(n-eulerphi(n))^2); \\ _Michel Marcus_, Aug 13 2019
%Y Cf. A051953, A001158, A091285.
%K nonn
%O 1,1
%A _Labos Elemer_, Feb 03 2004