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

1,2

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?

Links

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

Example

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

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 *)

Crossrefs

Cf. A008847, A000203, A097024, A028982.

Sequence in context: A206542 A206535 A017498 * A074387 A008848 A237182

Adjacent sequences: A097022 A097023 A097024 * A097026 A097027 A097028

Keyword

nonn

Author

Labos Elemer, Aug 24 2004

Extensions

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

Status

approved