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 A129131 #16 Sep 08 2022 08:45:30
%S A129131 4,2,6,3,8,5,9,7,10,11,12,13,14,17,15,19,16,23,18,29,20,31,21,37,22,
%T A129131 41,24,43,25,47,26,53,27,59,28,61,30,67,32,71,33,73,34,79,35,83,36,89,
%U A129131 38,97,39,101,40,103,42,107,44,109,45,113,46,127,48,131,49,137,50,139,51
%N A129131 Alternately write composite and prime numbers.
%C A129131 a(2n-1) = A073846(2n+1), a(2n) = A073846(2n).
%F A129131 a(n) = A066249(n) + 1. - _Filip Zaludek_, Dec 10 2016
%e A129131 Lowest composite number is 4, lowest prime number is 2, next composite number after 4 is 6, next prime number after 2 is 3 and so on.
%t A129131 f[n_]:=Module[{prs=Prime[Range[n]],comps},comps=Rest[Complement[Range[n+ Length[prs]+1], prs]];Riffle[comps,prs]] (* _Harvey P. Dale_, May 10 2011 *)
%o A129131 (Magma) P:=[ n : n in [2..150] | IsPrime(n) ]; C:=[ n : n in [2..70] | not IsPrime(n) ]; &cat[ [C[k], P[k] ]: k in [1..Minimum(#C, #P)] ]; // _Klaus Brockhaus_, Mar 31 2007
%Y A129131 Cf. A000040 (prime numbers), A002808 (composite numbers), A073846 (alternate nonprime and prime numbers).
%K A129131 nonn,easy
%O A129131 1,1
%A A129131 _Edwin F. Sampang_, Mar 30 2007
%E A129131 Edited and extended by _Klaus Brockhaus_, Mar 31 2007