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 A073920 a(1)=1 and then odd numbers rearranged such that a(n) is prime if n is composite else vice versa. 3
 1, 9, 15, 3, 21, 5, 25, 7, 11, 13, 27, 17, 33, 19, 23, 29, 35, 31, 39, 37, 41, 43, 45, 47, 53, 59, 61, 67, 49, 71, 51, 73, 79, 83, 89, 97, 55, 101, 103, 107, 57, 109, 63, 113, 127, 131, 65, 137, 139, 149, 151, 157, 69, 163, 167, 173, 179, 181, 75, 191, 77, 193, 197, 199 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Michel Marcus, Table of n, a(n) for n = 1..1000 Zak Seidov, A073920 [From Zak Seidov, Apr 15 2009] MATHEMATICA 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 *) PROG (PARI) nextcomposite(c) = until (!isprime(c), c +=2); c; 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 CROSSREFS Cf. A073921. Sequence in context: A298754 A166654 A131224 * A130119 A232395 A184048 Adjacent sequences:  A073917 A073918 A073919 * A073921 A073922 A073923 KEYWORD nonn AUTHOR Amarnath Murthy, Aug 19 2002 EXTENSIONS Extended and edited by T. D. Noe, Apr 13 2009 STATUS approved

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Last modified May 5 23:41 EDT 2021. Contains 343579 sequences. (Running on oeis4.)