A051572 a(1) = 5, a(n) = sigma(a(n-1)).

5, 6, 12, 28, 56, 120, 360, 1170, 3276, 10192, 24738, 61440, 196584, 491520, 1572840, 5433480, 20180160, 94859856, 355532800, 1040179456, 2143289344, 4966055344, 10092086208, 31800637440, 137371852800, 641012414823

Offset: 1

Author

Judson Neer

Keywords

• easy
• nonn

Links

• N. J. A. Sloane, Table of n, a(n) for n = 1..523
• G. L. Cohen and H. J. J. te Riele, Iterating the sum-of-divisors function, Experimental Mathematics, 5 (1996), pp. 91-100.
• Graeme L. Cohen and Herman J. J. te Riele, Iterating the Sum-of-Divisors Function, Experimental Mathematics, Vol. 5 (1996), No. 2, pp. 91-100.

Crossrefs

Cf. A000203, A007497, A257348.

Programs

Mathematica

NestList[Plus@@Divisors[#] &, 5, 25] (* Alonso del Arte, Apr 28 2011 *)

PARI

a=[5];for(i=2,10,a=concat(a,sigma(a[#a]))); a \\ Charles R Greathouse IV, Oct 03 2011

Python

from itertools import accumulate, repeat # requires Python 3.2 or higher
from sympy import divisor_sigma
A051572_list = list(accumulate(repeat(5,100), lambda x, _: divisor_sigma(x)))
# Chai Wah Wu, May 02 2015