A020900 - OEIS

%I #25 Feb 02 2020 11:03:16

%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 k-th prime < twice n-th 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_