This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A050438 #7 Oct 27 2023 22:00:44 %S A050438 26,33,38,39,42,49,52,55,56,60,68,69,70,74,77,78,80,84,88,93,94,95,98, %T A050438 100,105,106,110,115,118,119,121,124,125,126,130,133,138,140,141,145, %U A050438 146,152,154,155,156,159,160,162,164,165,170,174,176,180,183,184 %N A050438 Fourth-order composites. %H A050438 N. Fernandez, <a href="http://www.borve.org/primeness/FOP.html">An order of primeness, F(p)</a> %H A050438 N. Fernandez, <a href="/A006450/a006450.html">An order of primeness</a> [cached copy, included with permission of the author] %F A050438 Let C(n) be the n-th composite number, with C(1)=4. Then these are numbers C(C(C(C(n)))). %e A050438 C(C(C(C(8)))) = C(C(C(15))) = C(C(25)) = C(38) = 55. So 55 is in the sequence. %p A050438 C := remove(isprime,[$4..1000]): seq(C[C[C[C[n]]]],n=1..100); %Y A050438 Cf. A049076-A049081, A006450, A050435, A050436, A050439, A050440. %K A050438 easy,nonn %O A050438 1,1 %A A050438 Michael Lugo (mlugo(AT)thelabelguy.com), Dec 22 1999 %E A050438 More terms from _Asher Auel_ Dec 15 2000