A080402 Numbers k such that the sum of the squares of the divisors of k (A001157(k)) is not squarefree.

6, 7, 14, 15, 21, 24, 26, 27, 28, 30, 33, 34, 35, 36, 39, 41, 42, 43, 45, 46, 51, 54, 55, 56, 57, 60, 63, 65, 66, 69, 70, 74, 77, 78, 81, 82, 84, 85, 86, 87, 90, 91, 93, 94, 95, 96, 100, 102, 104, 105, 106, 107, 108, 110, 111, 112, 114, 115, 119, 120, 123, 125, 126, 129

Offset

1,1

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1000 from Harvey P. Dale)

Formula

mu(sigma_2(a(n))) = 0.

Mathematica

Do[s=MoebiusMu[DivisorSigma[2, n]]; If[Equal[s, 0], Print[n]], {n, 1, 1000}]
Select[Range[130], !SquareFreeQ[DivisorSigma[2, #]]&] (* Harvey P. Dale, Oct 10 2011 *)

Prog

(PARI) is(k) = !issquarefree(sigma(k, 2)); \\ Amiram Eldar, Aug 12 2024

Crossrefs

Cf. A001157, A005117, A013929, A065300, A080401 (complement).

Sequence in context: A288667 A319487 A283772 * A146308 A047589 A125996

Adjacent sequences: A080399 A080400 A080401 * A080403 A080404 A080405

Keyword

nonn

Author

Labos Elemer, Mar 19 2003

Status

approved