Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #7 Aug 18 2023 18:51:14
%S 1,3,12,60,360,2418,12544,58311,299520,2235324,14515200,122882760,
%T 1152592896,8365593600,81041688000,812657664000,8244072345600,
%U 78851793162240,701965262389248,5040363458592000,49476663690854400
%N a(n) = sigma(2*a(n-1)) for n>1 with a(1)=1.
%H Paul D. Hanna, <a href="/A180707/b180707.txt">Table of n, a(n) for n = 1..400</a>
%t NestList[DivisorSigma[1,2#]&,1,20] (* _Harvey P. Dale_, Jan 12 2018 *)
%o (PARI) a(n)=if(n==1,1,sigma(2*a(n-1)))
%K nonn
%O 1,2
%A _Paul D. Hanna_, Sep 18 2010