1,3

Robert Israel, Table of n, a(n) for n = 1..10000

For n=2, sigma(sigma(n)) = 4, so a(2) = 1.

seq(denom((numtheory:-sigma@@2)(n)/n), n=1..200); # Robert Israel, Aug 15 2018

(PARI) a(n) = denominator(sigma(sigma(n))/n);

Cf. A000203 (sigma), A051027.

Cf. A019278 (where a(n) = 1), A318059 (numerator).

Sequence in context: A030101 A162742 A081432 * A336652 A136655 A060819

Adjacent sequences: A318057 A318058 A318059 * A318061 A318062 A318063

nonn,frac

Michel Marcus, Aug 14 2018

approved