A073920 a(1)=1 and then odd numbers rearranged such that a(n) is prime if n is composite else vice versa.

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

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: A370674 A373330 A131224 * A130119 A346609 A232395

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