A087249 Squarefree numbers k such that sigma(k) is not squarefree.

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

Author

Labos Elemer, Sep 05 2003

Keywords

• nonn

Examples

			For k=7: sigma(7) = 2*2*2 = 8.
For k=10: sigma(10) = 1 + 2 + 5 + 10 = 18 = 2*3*3.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Complement of A065301 within A005117.

Cf. A065303, A087248.

Programs

Mathematica

Select[Range[150],SquareFreeQ[#]&&!SquareFreeQ[DivisorSigma[1,#]]&]  (* Harvey P. Dale, Feb 06 2011 *)

PARI

is(k) = issquarefree(k) && !issquarefree(sigma(k)); \\ Amiram Eldar, Jun 15 2024