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

1,1

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

Cf. A001157, A033766, A079488.

Sequence in context: A064006 A003095 A023362 * A299104 A090744 A041571

Adjacent sequences: A138610 A138611 A138612 * A138614 A138615 A138616

Keyword

nonn

Author

Zak Seidov, May 14 2008

Status

approved