A091286 Numbers n such that sigma_3(n) is divisible by square of cototient of n, while n is not a prime number.

15, 85, 259, 295, 391, 589, 799, 1111, 1717, 3193, 4171, 4369, 12361, 17473, 23533, 25429, 28243, 31351, 34477, 36181, 41407, 47989, 51143, 52537, 58103, 65641, 68377, 69541, 69919, 70453, 72619, 81121, 83131, 83767, 85069, 91759

Offset: 1

Author

Labos Elemer, Feb 03 2004

Keywords

• nonn

Examples

			n=15: cototient(15) = 7, sigma_3(15) = 3528 = 72 * 49.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A051953, A001158, A091285.

Programs

Mathematica

Do[s=DivisorSigma[3, n]/(n-EulerPhi[n])^2; If[IntegerQ[s]&&!PrimeQ[n], Print[n]], {n, 1, 100000}]

PARI

isok(n) = (n!=1) && !isprime(n) && !(sigma(n, 3)%(n-eulerphi(n))^2); \\ Michel Marcus, Aug 13 2019