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.

%I #19 Nov 06 2024 04:11:46

%S 2,6,8,7,10,13,14,19,23,29,20,37,22,43,47,53,27,61,30,71,73,79,35,89,

%T 97,101,103,107,44,113,46,131,137,139,149,151,54,163,167,173,58,181,

%U 62,193,197,199,66,223,227,229,233,239,75,251,257,263,269,271,82

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

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

%C Essentially the same as A088608. - _R. J. Mathar_, Feb 03 2014

%H Harvey P. Dale, <a href="/A236676/b236676.txt">Table of n, a(n) for n = 1..1000</a>

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

%t 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 *)

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

%Y Cf. A026234, A236675.

%K nonn

%O 1,1

%A _M. F. Hasler_, Jan 29 2014