OFFSET
1,2
COMMENTS
Perfect squares in the sequence are 1, 100, 1089, 13456, 21025, ...
LINKS
Robert G. Wilson v, Table of n, a(n) for n = 1..1510
EXAMPLE
730 is in the sequence because sigma(730) = 1332 = 11^3 + 1.
MAPLE
a:= proc(n) option remember; local k;
for k from 1+`if`(n=1, 0, a(n-1)) while (t->t<>
iroot(t, 3)^3)(numtheory[sigma](k)-1) do od; k
end:
seq(a(n), n=1..40); # Alois P. Heinz, Jul 04 2017
MATHEMATICA
fQ[n_] := ! PrimeQ@n && Block[{sd = DivisorSigma[1, n]}, IntegerQ[(sd - 1)^(1/3)]]; Select[Range@59323, fQ] (* Robert G. Wilson v, Jul 05 2017 *)
PROG
(PARI) isok(n) = ispower(sigma(n)-1, 3); \\ Michel Marcus, Jul 05 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Jul 04 2017
EXTENSIONS
More terms from Alois P. Heinz, Jul 04 2017
STATUS
approved