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a(n) = sigma(2*a(n-1)) for n>1 with a(1)=1.
2

%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