OFFSET
1,2
COMMENTS
Inspired by Allan C. Wechsler in seqfan list, Jan 07 2019.
Are 1 and 19 the only odd terms?
LINKS
Robert G. Wilson v, Table of n, a(n) for n = 1..70
MATHEMATICA
fQ[n_] := Mod[DivisorSigma[1, DivisorSigma[1, n^4]], n^2] == 0; Select[Range@ 476671, fQ]
PROG
(PARI) isok(n) = (sigma(sigma(n^4)) % n^2) == 0; \\ Michel Marcus, Jan 09 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Jan 08 2019
STATUS
approved