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A097025 Numbers n such that both sigma(n) and sigma(sigma(n)) are odd numbers. 1
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified September 28 06:22 EDT 2022. Contains 357063 sequences. (Running on oeis4.)