OFFSET
1,1
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
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
KEYWORD
nonn
AUTHOR
Labos Elemer, Sep 05 2003
STATUS
approved