A205597 Odd terms of A019278: odd n such that sigma(sigma(n))/n is an integer.

1, 15, 21, 1023, 29127, 550095, 355744082763

Offset: 1

Author

Jud McCranie, Feb 08 2012

Keywords

• nonn
• more

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

Examples

			15 is odd, sigma(15) = 24, sigma(24) = 60, and 60/15 is an integer.
		

Links

• Index entries for sequences where odd perfect numbers must occur, if they exist at all

Crossrefs

Cf. A000203, A019278, A019283, A228058, A323653.

Programs

Mathematica

Select[Range[1, 10^6, 2], Mod[Nest[DivisorSigma[1, #] &, #, 2], #] == 0 &] (* Michael De Vlieger, Mar 19 2021 *)

PARI

isok(n) = (n%2) && (denominator(sigma(sigma(n))/n) == 1); \\ Michel Marcus, Sep 27 2017