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A087249
Squarefree numbers k such that sigma(k) is not squarefree.
3
3, 6, 7, 10, 11, 14, 15, 17, 19, 21, 22, 23, 30, 31, 33, 34, 35, 38, 39, 42, 43, 46, 47, 51, 53, 55, 57, 58, 59, 62, 65, 66, 67, 69, 70, 71, 77, 78, 79, 82, 83, 85, 86, 87, 89, 91, 93, 94, 95, 97, 102, 103, 105, 106, 107, 110, 111, 114, 115, 118, 119, 123, 127, 129, 130
OFFSET
1,1
LINKS
EXAMPLE
For k=7: sigma(7) = 2*2*2 = 8.
For k=10: sigma(10) = 1 + 2 + 5 + 10 = 18 = 2*3*3.
MATHEMATICA
Select[Range[150], SquareFreeQ[#]&&!SquareFreeQ[DivisorSigma[1, #]]&] (* Harvey P. Dale, Feb 06 2011 *)
PROG
(PARI) is(k) = issquarefree(k) && !issquarefree(sigma(k)); \\ Amiram Eldar, Jun 15 2024
CROSSREFS
Complement of A065301 within A005117.
Sequence in context: A189139 A165496 A189204 * A188073 A286806 A297125
KEYWORD
nonn
AUTHOR
Labos Elemer, Sep 05 2003
STATUS
approved