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 Nov 08 2016 07:45:49
%S 1,2,4,3,6,5,8,7,9,10,12,11,14,13,15,16,18,17,20,19,21,22,24,23,25,26,
%T 27,28,30,29,32,31,33,34,35,36,38,37,39,40,42,41,44,43,45,46,48,47,49,
%U 50,51,52,54,53,55,56,57,58,60,59,62,61,63,64,65,66,68,67,69,70,72,71
%N Natural numbers with every odd prime p swapped with p+1.
%H G. C. Greubel, <a href="/A135761/b135761.txt">Table of n, a(n) for n = 1..10000</a>
%e Start with sequence of positive integers, then swap a(i) and a(i+1) if a(i) is an odd prime:
%e a(1)=1, a(2)=2, then a(3)=4 and a(4)=3 because p=3 is odd prime and p and p+1 are swapped.
%p f:= proc(n) if isprime(2*n+1) then (2*n+2,2*n+1) else (2*n+1,2*n+2) fi end proc:
%p map(f, [$0..100]); # _Robert Israel_, Nov 07 2016
%t r=Range[100];Do[If[PrimeQ[r[[i]]],{r[[i]],r[[i+1]]}={r[[i+1]],r[[i]]}],{i,3,97,2}];r
%Y Cf. A135762.
%K nonn
%O 1,2
%A _Zak Seidov_, Nov 27 2007