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%I #20 Oct 02 2019 04:32:40
%S 1,2,2,2,4,4,3,5,4,4,6,6,6,7,7,7,8,9,9,10,10,9,10,9,10,12,12,13,14,13,
%T 12,13,14,13,15,14,13,15,15,15,16,16,16,17,17,18,18,19,19,21,20,19,20,
%U 19,18,19,19,20,21,22,23,23,24,23,24,24,24,26,25,25,27,27,27,28,27,26
%N Number of primes between 2n and 4n.
%H T. D. Noe and Charles R Greathouse IV, <a href="/A101909/b101909.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) = A099802(2*n)-A099802(n). - _R. J. Mathar_, Oct 02 2019
%p A101909 := proc(n::integer)
%p numtheory[pi](4*n)-numtheory[pi](2*n) ;
%p end proc:
%p seq(A101909(n),n=1..100) ; # _R. J. Mathar_, Oct 02 2019
%t f[n_] := PrimePi[4n] - PrimePi[2n]; Table[ f[n], {n, 76}] (* _Robert G. Wilson v_, Feb 10 2005 *)
%o (PARI) bet2n4n(n) = { local(c,x,y); forstep(x=2,n,2, c=0; forprime(y=x+1,x+x-1, c++; ); print1(c",") ) }
%o (PARI) s=0;vector(100,n,s+=isprime(4*n-1)+isprime(4*n-3)-isprime(2*n-1)) \\ _Charles R Greathouse IV_, Mar 12 2012
%Y Cf. A101947, A101983, A101984, A101985.
%K easy,nonn
%O 1,2
%A _Cino Hilliard_, Jan 28 2005
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