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A138613 a(n) = sigma_2(a(n-1)): sum of squares of divisors of a(n-1) with a(1)=2.

%I A138613 #8 Jul 17 2020 22:34:41
%S A138613 2,5,26,850,943950,1318281510000,2755017380686402917800000,
%T A138613 10751241448828145947377163154256928532205365443200,
%U A138613 186653497320711305248493130820948403307540799528422902350199946673943964748877201760000000000000000
%N A138613 a(n) = sigma_2(a(n-1)): sum of squares of divisors of a(n-1) with a(1)=2.
%C A138613 Are all following terms divisible by 10?
%H A138613 Robert Gerbicz, <a href="/A138613/b138613.txt">Table of n, a(n) for n = 1..12</a>
%F A138613 a(n>=1) = sigma_2(a(n-1)) = A001157(a(n-1)), a(1)=2.
%e A138613 a(1)=2, divisors of 2: {1,2}, hence a(2)=1^2+2^2=5;
%e A138613 a(2)=5, divisors of 5: {1,5}, hence a(3)=1^2+5^2=26;
%e A138613 a(3)=26, divisors of 26: {1,2,13,26}, hence a(4)=1^2+2^2+13^2+26^2=850.
%t A138613 NestList[DivisorSigma[2, # ]&,2,9]
%Y A138613 Cf. A001157, A033766, A079488.
%K A138613 nonn
%O A138613 1,1
%A A138613 _Zak Seidov_, May 14 2008