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 A138561 #5 Dec 05 2013 19:57:15
%S A138561 1,2,3,4,6,8,9,5,7,11,10,12,14,15,16,18,13,17,19,23,29,20,21,22,24,25,
%T A138561 26,27,28,31,37,41,43,47,53,59,30,32,33,34,35,36,38,39,40,61,67,71,73,
%U A138561 79,83,89,97,101,103,107,42,44,45,46,48,49,50,51,52,54,109,113,127,131
%N A138561 Start with the list {1}; for each n >= 1, append p(n) primes followed by c(n) composite numbers, where p(n) is the n-th prime and c(n) is the n-th composite number.
%C A138561 A rearrangement of the natural numbers.
%e A138561 We start with 1; p(1) = 2 is the first prime and the next two terms are 2,3 while c(1)=4 is the first composite number and the next four terms are 4,6,8,9 and so on.
%K A138561 easy,nonn
%O A138561 1,2
%A A138561 _Amarnath Murthy_, Mar 25 2008
%E A138561 More terms from _Sean A. Irvine_, Apr 13 2010