A236676 - OEIS

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

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

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

FORMULA

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

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 *)

PROG

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

CROSSREFS

Cf. A026234, A236675.

Sequence in context: A182586 A011224 A272097 * A021376 A243311 A093888

Adjacent sequences: A236673 A236674 A236675 * A236677 A236678 A236679

KEYWORD

nonn,changed

AUTHOR

M. F. Hasler, Jan 29 2014

STATUS

approved