%I #33 Mar 20 2021 10:55:04
%S 1,15,21,1023,29127,550095,355744082763
%N Odd terms of A019278: odd n such that sigma(sigma(n))/n is an integer.
%C a(8) > 4*10^12, if it exists. - _Giovanni Resta_, Feb 26 2020
%C 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
%H <a href="/index/O#opnseqs">Index entries for sequences where odd perfect numbers must occur, if they exist at all</a>
%e 15 is odd, sigma(15) = 24, sigma(24) = 60, and 60/15 is integer.
%t Select[Range[1, 10^6, 2], Mod[Nest[DivisorSigma[1, #] &, #, 2], #] == 0 &] (* _Michael De Vlieger_, Mar 19 2021 *)
%o (PARI) isok(n) = (n%2) && (denominator(sigma(sigma(n))/n) == 1); \\ _Michel Marcus_, Sep 27 2017
%Y Cf. A000203, A019278, A019283, A228058, A323653.
%K nonn,more
%O 1,2
%A _Jud McCranie_, Feb 08 2012
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