A107347 - OEIS

%I #15 Sep 02 2024 04:28:53

%S 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,

%T 13,13,14,13,14,15,15,15,16,16,17,18,19,19,20,19,18,18,19,20,21,22,23,

%U 23,23,24,25,25,25,26,27,26,27,28,28,28,28,28,29,30,30,30,31,32,32,32,32

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

%H N. J. A. Sloane, <a href="/A107347/b107347.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = A294602(prime(n)). - _R. J. Mathar_, Dec 17 2017

%F a(n) = n-1-pi(prime(n)/2). - _N. J. A. Sloane_, Nov 04 2020

%p A107347 := n -> n-1-pi(ithprime(n)/2);

%p [seq(A107347(n),n=1..100)]; # _N. J. A. Sloane_, Nov 04 2020

%t 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 *)

%K nonn,easy

%O 1,6

%A _Giovanni Teofilatto_, May 23 2005

%E Corrected and extended by _Robert G. Wilson v_, May 28 2005