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%I #7 Aug 18 2023 18:56:03
%S 1,4,28,224,2016,20160,243840,2545920,30905280,388908036,3622707200,
%T 44117911764,387754160640,5065246517760,67333642371072,
%U 829993839244416,9887956143636480,150394118343146496,1797812804753817600
%N a(n) = sigma(3*a(n-1)) for n>1 with a(1)=1.
%H Paul D. Hanna, <a href="/A180708/b180708.txt">Table of n, a(n) for n = 1..300</a>
%t NestList[DivisorSigma[1,3#]&,1,20] (* _Harvey P. Dale_, Aug 18 2023 *)
%o (PARI) a(n)=if(n==1,1,sigma(3*a(n-1)))
%Y Cf. A180707.
%K nonn
%O 1,2
%A _Paul D. Hanna_, Sep 18 2010
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