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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
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
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
KEYWORD
nonn
AUTHOR
Labos Elemer, Aug 24 2004
EXTENSIONS
Edited and extended by Robert G. Wilson v, Aug 27 2004
STATUS
approved