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A205597
Odd terms of A019278: odd n such that sigma(sigma(n))/n is an integer.
2
1, 15, 21, 1023, 29127, 550095, 355744082763
OFFSET
1,2
COMMENTS
a(8) > 4*10^12, if it exists. - Giovanni Resta, Feb 26 2020
First five terms are squarefree. Sigma(sigma(n))/n ratios for these seven known terms are: 1, 4, 3, 4, 4, 6, 4. - Antti Karttunen, Mar 19 2021
EXAMPLE
15 is odd, sigma(15) = 24, sigma(24) = 60, and 60/15 is integer.
MATHEMATICA
Select[Range[1, 10^6, 2], Mod[Nest[DivisorSigma[1, #] &, #, 2], #] == 0 &] (* Michael De Vlieger, Mar 19 2021 *)
PROG
(PARI) isok(n) = (n%2) && (denominator(sigma(sigma(n))/n) == 1); \\ Michel Marcus, Sep 27 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Jud McCranie, Feb 08 2012
STATUS
approved