A107347 Number of even semiprimes strictly between prime(n) and 2*prime(n).

0, 1, 1, 1, 1, 2, 2, 3, 3, 3, 4, 4, 4, 5, 5, 6, 6, 7, 7, 8, 9, 9, 9, 9, 9, 10, 11, 11, 12, 13, 12, 13, 13, 14, 13, 14, 15, 15, 15, 16, 16, 17, 18, 19, 19, 20, 19, 18, 18, 19, 20, 21, 22, 23, 23, 23, 24, 25, 25, 25, 26, 27, 26, 27, 28, 28, 28, 28, 28, 29, 30, 30, 30, 31, 32, 32, 32, 32

Offset: 1

Author

Giovanni Teofilatto, May 23 2005

Keywords

• nonn
• easy

Links

• N. J. A. Sloane, Table of n, a(n) for n = 1..10000

Programs

Maple

A107347 := n -> n-1-pi(ithprime(n)/2);
[seq(A107347(n),n=1..100)]; # N. J. A. Sloane, Nov 04 2020

Mathematica

fQ[n_] := Plus @@ Last /@ FactorInteger[n] == 2; f[n_] := Count[fQ /@ Range[Prime[n] + 1, Prime[n + 1] - 1], True]; Table[ f[n], {n, 78}] (* Robert G. Wilson v, May 28 2005 *)

Formula

a(n) = A294602(prime(n)). - R. J. Mathar, Dec 17 2017

a(n) = n-1-pi(prime(n)/2). - N. J. A. Sloane, Nov 04 2020

Extensions

Corrected and extended by Robert G. Wilson v, May 28 2005