

A020900


Greatest k such that kth prime < twice nth prime.


13



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, 46, 46, 47, 47, 48, 54, 55, 58, 59, 62, 62, 65, 66, 67, 68, 71, 72, 75, 76, 77, 78, 82, 86, 87, 88, 90, 91, 92, 95, 97, 99, 99, 100, 101, 102, 103, 106, 112
(list;
graph;
refs;
listen;
history;
text;
internal format)



OFFSET

1,1


LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000


FORMULA

a(n) = A000720(A100484(n)).  Michel Marcus, Feb 02 2020


EXAMPLE

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


MATHEMATICA

PrimePi[NextPrime[#, 1]]&/@(2Prime[Range[70]]) (* Harvey P. Dale, Jul 05 2012 *)


PROG

(PARI) a(n) = primepi(2*prime(n)); \\ Michel Marcus, Oct 25 2017; Feb 02 2020


CROSSREFS

Cf. A000720 (pi(n)), A100484 (2*prime(n)).
Cf. A020901, A059788, A060265.
Cf. A102820 (first differences).
Sequence in context: A233133 A186541 A207432 * A333309 A282697 A002479
Adjacent sequences: A020897 A020898 A020899 * A020901 A020902 A020903


KEYWORD

nonn


AUTHOR

Clark Kimberling


STATUS

approved



