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%I #15 Jun 17 2020 04:18:04
%S 1,2,4,9,13,16,25,37,43,49,61,64,67,73,81,97,109,121,151,157,163,169,
%T 181,193,211,225,229,241,277,283,289,313,331,337,361,373,397,409,421,
%U 433,457,487,523,529,541,547,577,601,613,625,631,661,673,691,709,729,733,751,757,787,823,829,841,853,877
%N Numbers k such that sigma(sigma(k)) < 2*k + 1.
%H Amiram Eldar, <a href="/A320514/b320514.txt">Table of n, a(n) for n = 1..10000</a>
%p isA320514 := n -> sigma(sigma(n)) < 2*n+1:
%p select(isA320514, [$1..1000]);
%t Select[Range[1000], DivisorSigma[1, DivisorSigma[1, #]] < 2*# + 1 &] (* _Vaclav Kotesovec_, Oct 14 2018 *)
%o (PARI) isok(n) = sigma(sigma(n)) < 2*n + 1; \\ _Michel Marcus_, Oct 14 2018
%Y Cf. A000203, A320515.
%K nonn
%O 1,2
%A _Peter Luschny_, Oct 14 2018
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