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%I #4 Oct 02 2012 15:32:08
%S 1,3,28,504,26520,3090528,690908400,432222336000,543025101265920,
%T 1255553120600064000,6002949274226395545600,
%U 65900815978203369461760000,1290586365875976383280969154560,48847915039427778415352397535641600
%N a(n) = sigma(2^(n-1)*a(n-1)) for n>1 with a(1)=1.
%C Conjecture: sigma(n) | a(n) for n>=1.
%t nxt[{a_,b_}]:={a+1,DivisorSigma[1,2^a*b]}; Transpose[NestList[nxt,{1,1},20]][[2]] (* _Harvey P. Dale_, Oct 02 2012 *)
%o (PARI) a(n)=if(n==1,1,sigma(2^(n-1)*a(n-1)))
%K nonn
%O 1,2
%A _Paul D. Hanna_, Sep 18 2010
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