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%I #13 Oct 27 2023 22:00:44
%S 16,21,25,26,28,33,36,38,39,42,48,49,50,52,55,56,57,60,64,68,69,70,72,
%T 74,77,78,80,84,87,88,90,93,94,95,98,100,104,105,106,110,111,115,117,
%U 118,119,121,122,124,125,126,130,133,135,138,140,141,145,146,147
%N Third-order composites.
%H N. Fernandez, <a href="http://www.borve.org/primeness/FOP.html">An order of primeness, F(p)</a>
%H N. Fernandez, <a href="/A006450/a006450.html">An order of primeness</a> [cached copy, included with permission of the author]
%F Let C(n) be the n-th composite number, with C(1)=4. Then these are numbers C(C(C(n))).
%e C(C(C(8))) = C(C(15)) = C(25) = 38. So 38 is in the sequence.
%p C := remove(isprime,[$4..1000]): seq(C[C[C[C[n]]]],n=1..100);
%t Nest[Values@ KeySelect[MapIndexed[First@ #2 -> #1 &, #], CompositeQ] &, Select[Range@ 150, CompositeQ], 2] (* _Michael De Vlieger_, Jul 22 2017 *)
%Y Cf. A049076, A049077, A049078, A049079, A049080, A049081, A006450, A050435, A050438, ...
%K easy,nonn
%O 1,1
%A Michael Lugo (mlugo(AT)thelabelguy.com), Dec 22 1999
%E More terms from _Asher Auel_ Dec 15 2000
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