Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #10 Oct 03 2015 23:21:55
%S 1,2,0,3,4,5,6,8,9,7,10,11,12,14,15,13,16,17,18,20,21,19,22,24,25,26,
%T 27,23,28,29,30,32,33,34,35,31,36,38,39,37,40,41,42,44,45,43,46,48,49,
%U 50,51,47,52,54,55,56,57,53,58,59,60,62,63,64,65,61,66,68,69,67,70,71,72
%N 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. a(2) = 1. a(4) = 0.
%C This is a permutation of the nonnegative integers.
%p 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: A139637 := proc(n) local k; if n = 2 then 1; elif n = 4 then 0 ; else if isprime(n) then k := A049084(n) ; A000040(k-1) ; else k := A066246(n) ; A002808(k-1) ; fi ; fi ; end: seq(A139637(n),n=2..160) ; # _R. J. Mathar_, May 12 2008
%Y Cf. A139636.
%K nonn
%O 2,2
%A _Leroy Quet_, Apr 28 2008
%E More terms from _R. J. Mathar_, May 12 2008