%I #2 Mar 30 2012 17:26:30

%S 2,1,2,5,4,7,6,9,10,11,10,13,12,15,16,17,16,19,18,21,22,23,22,25,26,

%T 27,28,29,28,31,30,33,34,35,36,37,36,39,40,41,40,43,42,45,46,47,46,49,

%U 50,51,52,53,52,55,56,57,58,59,58,61,60,63,64,65,66,67,66,69,70,71,70,73

%N a(n)=n-1 if n is prime, otherwise a(n)=n+1.

%t Table[If[PrimeQ[n],n-1,n+1],{n,1,200}]

%Y Cf. A068519, A167480, A175027.

%K nonn

%O 1,1

%A _Zak Seidov_, Nov 04 2009