A138613 a(n) = sigma_2(a(n-1)): sum of squares of divisors of a(n-1) with a(1)=2.
2, 5, 26, 850, 943950, 1318281510000, 2755017380686402917800000, 10751241448828145947377163154256928532205365443200, 186653497320711305248493130820948403307540799528422902350199946673943964748877201760000000000000000
Offset: 1
Keywords
Examples
a(1)=2, divisors of 2: {1,2}, hence a(2)=1^2+2^2=5;
a(2)=5, divisors of 5: {1,5}, hence a(3)=1^2+5^2=26;
a(3)=26, divisors of 26: {1,2,13,26}, hence a(4)=1^2+2^2+13^2+26^2=850.
Links
- Robert Gerbicz, Table of n, a(n) for n = 1..12
Programs
-
Mathematica
NestList[DivisorSigma[2, # ]&,2,9]
Formula
a(n>=1) = sigma_2(a(n-1)) = A001157(a(n-1)), a(1)=2.
Comments