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%I #5 Mar 30 2012 18:50:20
%S 2,2,2,3,3,3,3,3,3,5,5,5,5,5,5,7,7,7,7,7,7,11,11,11,11,11,11,11,11,13,
%T 13,13,13,13,13,13,13,13,13,17,17,17,17,17,17,19,19,19,19,19,19,19,19,
%U 19,19,19,19,23,23,23,23,23,23,23,23,29,29
%N Smallest prime p such that pi(n) <= pi(p)*2, where pi(n) is the number of primes <= n, A000720.
%C a(n) separates the primes <= n in two sets {q<=a(n)} and {q<a(n)<=n} which differ in size by not more than 1.
%e Primes <= 20: {2,3,5,7,11,13,17,19}, therefore pi(20)=8 and from 8<=pi(7)*2=4*2 follows a(20)=7.
%Y Cf. A007917, A000720, A000040.
%K nonn
%O 2,1
%A _Reinhard Zumkeller_, Apr 18 2002
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