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A080402
Numbers k such that the sum of the squares of the divisors of k (A001157(k)) is not squarefree.
3
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
KEYWORD
nonn
AUTHOR
Labos Elemer, Mar 19 2003
STATUS
approved