A073881 - OEIS

%I #16 Oct 08 2024 07:27:02

%S 1,3,7,7,13,13,19,19,19,19,29,29,37,37,37,37,43,43,53,53,53,53,61,61,

%T 61,61,61,61,71,71,79,79,79,79,79,79,89,89,89,89,101,101,107,107,107,

%U 107,113,113,113,113,113,113,131,131,131,131,131,131,139,139,151,151

%N a(n) = smallest number m (obviously prime) such that pi(m) = 2*pi(n).

%H Robert Israel, <a href="/A073881/b073881.txt">Table of n, a(n) for n = 1..10000</a>

%p 1,seq(ithprime(2*numtheory:-pi(n)),n=2..100); # _Robert Israel_, Oct 07 2024

%o (PARI) a(n) = my(k=1, nb=primepi(n)); while(primepi(k) != 2*nb, k++); k; \\ _Michel Marcus_, Nov 25 2023

%Y Cf. A000720.

%K nonn

%O 1,2

%A _Amarnath Murthy_, Aug 16 2002

%E More terms from _Jason Earls_, Aug 22 2002