%I
%S 2,3,4,6,8,9,11,12,14,16,18,21,22,23,24,27,30,30,32,34,34,37,38,40,44,
%T 46,46,47,47,48,54,55,58,59,62,62,65,66,67,68,71,72,75,76,77,78,82,86,
%U 87,88,90,91,92,95,97,99,99,100,101,102,103,106,112
%N Greatest k such that kth prime < twice nth prime.
%H Harvey P. Dale, <a href="/A020900/b020900.txt">Table of n, a(n) for n = 1..1000</a>
%F a(n) = A000720(A100484(n)).  _Michel Marcus_, Feb 02 2020
%e 4th prime is 7, twice the 4th prime is 14, the greatest prime < 14 is 13 that is the 6th prime, hence, a(4) = 6.  _Bernard Schott_, Feb 02 2020
%t PrimePi[NextPrime[#,1]]&/@(2Prime[Range[70]]) (* _Harvey P. Dale_, Jul 05 2012 *)
%o (PARI) a(n) = primepi(2*prime(n)); \\ _Michel Marcus_, Oct 25 2017; Feb 02 2020
%Y Cf. A000720 (pi(n)), A100484 (2*prime(n)).
%Y Cf. A020901, A059788, A060265.
%Y Cf. A102820 (first differences).
%K nonn
%O 1,1
%A _Clark Kimberling_
