A097025 Numbers n such that both sigma(n) and sigma(sigma(n)) are odd numbers.

1, 81, 400, 32400, 195938, 224450, 1705636, 3648100, 13645088, 15870978, 18180450, 29184800, 138156516, 295496100, 1055340196, 1105252128, 1476326929, 2263475776, 2323432804, 2363968800, 2592846400, 2661528100, 2686005218, 2917410498, 3564550178

Offset: 1

Author

Labos Elemer, Aug 24 2004

Keywords

• nonn

Comments

Only members of A028982 are candidates. - Robert G. Wilson v, Aug 27 2004

Question: iterating sigma()=A000203, how many iterates can be odd numbers?

Examples

			n = 1910^2, sigma(n) = 2821^2, sigma(sigma(n)) = 10357983.
		

Links

• Donovan Johnson, Table of n, a(n) for n = 1..500

Crossrefs

Cf. A008847, A000203, A097024, A028982.

Programs

Mathematica

t = Sort[ Flatten[ Table[{n^2, 2n^2}, {n, 36650}]]]; a = {}; Do[ If[ OddQ[ DivisorSigma[1, DivisorSigma[1, t[[n]] ]]], AppendTo[a, t[[n]] ]], {n, 2*10^6}]; a (* Robert G. Wilson v, Aug 27 2004 *)

Extensions

Edited and extended by Robert G. Wilson v, Aug 27 2004