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a(n) is the least integer k such that sigma(sigma(k)) >= n*k where sigma is A000203, the sum of divisors.
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%I #51 Feb 27 2020 05:57:22

%S 1,2,6,6,24,24,24,60,120,480,540,1560,2520,10920,27720,30240,191520,

%T 360360,360360,3243240,5765760,28828800,109549440,438197760,766846080,

%U 3834230400,9081072000,32974381440,147516969600,880887047040,2802822422400

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

%C From _Daniel Suteu_, Oct 10 2019: (Start)

%C a(32) <= 14814918518400,

%C a(33) <= 64464915715200,

%C a(34) <= 709114072867200,

%C a(35) <= 9881550651772800,

%C a(36) <= 76648784785372800,

%C a(37) <= 2376112328346556800. (End)

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

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

%Y Cf. A051027, A272930.

%K nonn,more

%O 1,2

%A _Michel Marcus_, Oct 07 2019

%E a(23)-a(29) from _Giovanni Resta_, Oct 07 2019

%E a(30)-a(31) from _Giovanni Resta_, Feb 27 2020