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%I #14 Mar 18 2024 03:27:22
%S 81,1681,3481,5041,7921,17161,27889,29929,83521,146689,279841,491401,
%T 552049,579121,635209,683929,703921,707281,829921,1190281,1203409,
%U 1352569,1481089,1885129,2036329,2211169,2430481,2505889,3279721,3411409,3523129,3568321,3728761
%N Numbers k that require four iterations of the sigma function to be >= 2*k.
%C Most of the early terms are the square of a prime or the fourth power of a prime.
%H Amiram Eldar, <a href="/A107914/b107914.txt">Table of n, a(n) for n = 1..500</a>
%e sigma(sigma(sigma(81))) = 160 but sigma(sigma(sigma(sigma(81)))) = 378, so 81 is in the sequence.
%o (PARI) is(n) = {my(m = n, nn = 2*n, c = 0); while(m < nn, m = sigma(m); c++); c == 4;} \\ _Amiram Eldar_, Mar 18 2024
%Y Cf. A000203, A055020, A055021, A107912, A107913.
%K nonn
%O 1,1
%A _Jud McCranie_, May 27 2005
%E Offset 1 by _Michel Marcus_, Apr 25 2020
%E More terms from _Amiram Eldar_, Mar 18 2024
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