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%I #12 Sep 22 2016 05:18:32
%S 1,4,8,32,32,128,128,256,512,1024,1024,4096,4096,8192,16384,32768,
%T 32768,65536,131072,131072,262144,262144,524288,524288,1048576,
%U 1048576,1048576,2097152,2097152,4194304,4194304,4194304,4194304,8388608
%N a(n) = A000203(A046528(n)): sigma of those numbers whose sigma is a power of 2, in order of appearance.
%C Observe that certain powers of 2 do not arise as sum of divisors of something: 2,16,64,2048. Are there more? Yes, see A094505 and A078426.
%F a(n) = 2^A048947(n). - _R. J. Mathar_, Sep 22 2016
%t {ta=Table[0, {100}], u=1}; Do[If[IntegerQ[Log[2, DivisorSigma[1, n]]], Print[n];ta[[u]]=n;u=u+1], {n, 1, 100000000}] DivisorSigma[1, ta]
%o (PARI) isok(n) = (n==1) || (ispower(sigma(n), , &r) && (r==2));
%o for(n=1, 1e7, if(isok(n), print1(sigma(n)", "))) \\ _Altug Alkan_, Nov 01 2015
%Y Cf. A000203, A046528, A078426, A094505.
%K nonn
%O 1,2
%A _Labos Elemer_, Jun 02 2004
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