A236676 The n-th term is the n-th composite if n is prime, and the n-th prime if n is not prime.

2, 6, 8, 7, 10, 13, 14, 19, 23, 29, 20, 37, 22, 43, 47, 53, 27, 61, 30, 71, 73, 79, 35, 89, 97, 101, 103, 107, 44, 113, 46, 131, 137, 139, 149, 151, 54, 163, 167, 173, 58, 181, 62, 193, 197, 199, 66, 223, 227, 229, 233, 239, 75, 251, 257, 263, 269, 271, 82

Offset: 1

Author

M. F. Hasler, Jan 29 2014

Keywords

• nonn

Comments

See A097457 for the (maybe more natural) variant where "n-th composite"(=A002808(n)) is replaced by "n-th nonprime" (=A018252(n)).

Essentially the same as A088608. - R. J. Mathar, Feb 03 2014

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A026234, A236675.

Programs

Mathematica

Module[{nn=100,prs,cmps,len},cmps=Select[Range[nn],CompositeQ];len= Length[ cmps];prs=Prime[Range[len]];Table[If[PrimeQ[n],cmps[[n]],prs[[n]]],{n,len}]] (* Harvey P. Dale, Dec 27 2022 *)

PARI

A236676 = n->if(isprime(n),A002808(n),prime(n))

Formula

a(n) = A002808(n) if n is prime, a(n) = A000040(n) else.