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 A139636 #13 Jun 26 2022 11:34:09
%S A139636 3,5,6,7,8,11,9,10,12,13,14,17,15,16,18,19,20,23,21,22,24,29,25,26,27,
%T A139636 28,30,31,32,37,33,34,35,36,38,41,39,40,42,43,44,47,45,46,48,53,49,50,
%U A139636 51,52,54,59,55,56,57,58,60,61,62,67,63,64,65,66,68,71,69,70,72,73,74
%N A139636 If n = the k-th prime, then a(n) = the (k+1)th prime. If n = the k-th composite, then a(n) = the (k+1)th composite.
%C A139636 This is a permutation of the positive integers sans 1,2,4.
%H A139636 Harvey P. Dale, <a href="/A139636/b139636.txt">Table of n, a(n) for n = 2..1000</a>
%p A139636 A000040 := proc(n) ithprime(n) ; end: A002808 := proc(n) local a; if n = 1 then 4; else for a from A002808(n-1)+1 do if not isprime(a) then RETURN(a) ; fi ; od: fi ; end: A066246 := proc(n) local k ; if isprime(n) then 0 ; else for k from 1 do if A002808(k) = n then RETURN(k) ; fi ; od: fi ; end: A049084 := proc(n) if not isprime(n) then 0; else numtheory[pi](n) ; fi ; end: A139636 := proc(n) local k; if isprime(n) then k := A049084(n) ; RETURN(A000040(k+1)) ; else k := A066246(n) ; RETURN(A002808(k+1)) ; fi ; end: seq(A139636(n),n=2..160) ; # _R. J. Mathar_, May 12 2008
%t A139636 npc[n_]:=Module[{k=1},If[PrimeQ[n],NextPrime[n],While[PrimeQ[n+k],k++];n+k]]; Array[npc,80,2] (* _Harvey P. Dale_, Jun 26 2022 *)
%Y A139636 Cf. A139637.
%K A139636 nonn
%O A139636 2,1
%A A139636 _Leroy Quet_, Apr 28 2008
%E A139636 More terms from _R. J. Mathar_, May 12 2008