A327630 a(n) is the least integer k such that sigma(sigma(k)) >= n*k where sigma is A000203, the sum of divisors.

1, 2, 6, 6, 24, 24, 24, 60, 120, 480, 540, 1560, 2520, 10920, 27720, 30240, 191520, 360360, 360360, 3243240, 5765760, 28828800, 109549440, 438197760, 766846080, 3834230400, 9081072000, 32974381440, 147516969600, 880887047040, 2802822422400

Offset: 1

Author

Michel Marcus, Oct 07 2019

Keywords

• nonn
• more

Comments

From Daniel Suteu, Oct 10 2019: (Start)

a(32) <= 14814918518400,

a(33) <= 64464915715200,

a(34) <= 709114072867200,

a(35) <= 9881550651772800,

a(36) <= 76648784785372800,

a(37) <= 2376112328346556800. (End)

Crossrefs

Cf. A000203, A007539, A023199, A098221, A019278.

Cf. A051027, A272930.

Programs

PARI

a(n) = my(k=1); while (sigma(sigma(k))/k < n, k++); k;

Extensions

a(23)-a(29) from Giovanni Resta, Oct 07 2019

a(30)-a(31) from Giovanni Resta, Feb 27 2020