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%I #19 Apr 02 2017 10:14:37
%S 1,9,15,3,21,5,25,7,11,13,27,17,33,19,23,29,35,31,39,37,41,43,45,47,
%T 53,59,61,67,49,71,51,73,79,83,89,97,55,101,103,107,57,109,63,113,127,
%U 131,65,137,139,149,151,157,69,163,167,173,179,181,75,191,77,193,197,199
%N a(1)=1 and then odd numbers rearranged such that a(n) is prime if n is composite else vice versa.
%H Michel Marcus, <a href="/A073920/b073920.txt">Table of n, a(n) for n = 1..1000</a>
%H Zak Seidov, <a href="http://zak08.livejournal.com/11403.html">A073920</a> [From _Zak Seidov_, Apr 15 2009]
%t a = {1}; Do[k = 3; While[Nand[! MemberQ[a, k], Xor @@ Map[PrimeQ, {k, n}]], k += 2]; AppendTo[a, k], {n, 2, 64}]; a (* _Michael De Vlieger_, Mar 30 2017 *)
%o (PARI) nextcomposite(c) = until (!isprime(c), c +=2); c;
%o lista(nn) = {print1(1, ", "); p = 2; c = 1; for (n=2, nn, if (isprime(n), c = nextcomposite(c); print1(c, ", "), p = nextprime(p+1); print1(p, ", ")););} \\ _Michel Marcus_, Apr 01 2017
%Y Cf. A073921.
%K nonn
%O 1,2
%A _Amarnath Murthy_, Aug 19 2002
%E Extended and edited by _T. D. Noe_, Apr 13 2009
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