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%I #26 Dec 05 2021 19:12:44
%S 5,6,12,28,56,120,360,1170,3276,10192,24738,61440,196584,491520,
%T 1572840,5433480,20180160,94859856,355532800,1040179456,2143289344,
%U 4966055344,10092086208,31800637440,137371852800,641012414823
%N a(1) = 5, a(n) = sigma(a(n-1)).
%H N. J. A. Sloane, <a href="/A051572/b051572.txt">Table of n, a(n) for n = 1..523</a>
%H G. L. Cohen and H. J. J. te Riele, <a href="http://projecteuclid.org/euclid.em/1047565640">Iterating the sum-of-divisors function</a>, Experimental Mathematics, 5 (1996), pp. 91-100.
%H Graeme L. Cohen and Herman J. J. te Riele, <a href="http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.52.868">Iterating the Sum-of-Divisors Function</a>, Experimental Mathematics, Vol. 5 (1996), No. 2, pp. 91-100.
%t NestList[Plus@@Divisors[#] &, 5, 25] (* _Alonso del Arte_, Apr 28 2011 *)
%o (PARI) a=[5];for(i=2,10,a=concat(a,sigma(a[#a]))); a \\ _Charles R Greathouse IV_, Oct 03 2011
%o (Python)
%o from itertools import accumulate, repeat # requires Python 3.2 or higher
%o from sympy import divisor_sigma
%o A051572_list = list(accumulate(repeat(5,100), lambda x, _: divisor_sigma(x)))
%o # _Chai Wah Wu_, May 02 2015
%Y Cf. A000203, A007497, A257348.
%K easy,nonn
%O 1,1
%A _Judson Neer_
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