OFFSET
1,1
COMMENTS
Are all following terms divisible by 10?
LINKS
Robert Gerbicz, Table of n, a(n) for n = 1..12
FORMULA
a(n>=1) = sigma_2(a(n-1)) = A001157(a(n-1)), a(1)=2.
EXAMPLE
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.
MATHEMATICA
NestList[DivisorSigma[2, # ]&, 2, 9]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zak Seidov, May 14 2008
STATUS
approved