OFFSET
1,1
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
EXAMPLE
sigma(8) = 15 but sigma(sigma(8)) = 25, so 8 is in the sequence.
MATHEMATICA
ds2Q[n_]:=Module[{ds1=DivisorSigma[1, n]}, ds1<2n<=DivisorSigma[1, ds1]]; Select[Range[120], ds2Q] (* Harvey P. Dale, Feb 13 2011 *)
PROG
(PARI) is(n) = {my(m = n, nn = 2*n, c = 0); while(m < nn, m = sigma(m); c++); c == 2; } \\ Amiram Eldar, Mar 18 2024
CROSSREFS
KEYWORD
nonn
AUTHOR
Jud McCranie, May 27 2005
EXTENSIONS
Offset 1 by Michel Marcus, Apr 25 2020
STATUS
approved