

A101909


Number of primes between 2n and 4n.


5



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, 12, 13, 14, 13, 15, 14, 13, 15, 15, 15, 16, 16, 16, 17, 17, 18, 18, 19, 19, 21, 20, 19, 20, 19, 18, 19, 19, 20, 21, 22, 23, 23, 24, 23, 24, 24, 24, 26, 25, 25, 27, 27, 27, 28, 27, 26
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OFFSET

1,2


LINKS

T. D. Noe and Charles R Greathouse IV, Table of n, a(n) for n = 1..10000


FORMULA

a(n) = A099802(2*n)A099802(n).  R. J. Mathar, Oct 02 2019


MAPLE

A101909 := proc(n::integer)
numtheory[pi](4*n)numtheory[pi](2*n) ;
end proc:
seq(A101909(n), n=1..100) ; # R. J. Mathar, Oct 02 2019


MATHEMATICA

f[n_] := PrimePi[4n]  PrimePi[2n]; Table[ f[n], {n, 76}] (* Robert G. Wilson v, Feb 10 2005 *)


PROG

(PARI) bet2n4n(n) = { local(c, x, y); forstep(x=2, n, 2, c=0; forprime(y=x+1, x+x1, c++; ); print1(c", ") ) }
(PARI) s=0; vector(100, n, s+=isprime(4*n1)+isprime(4*n3)isprime(2*n1)) \\ Charles R Greathouse IV, Mar 12 2012


CROSSREFS

Cf. A101947, A101983, A101984, A101985.
Sequence in context: A259981 A070705 A216326 * A215676 A130872 A087627
Adjacent sequences: A101906 A101907 A101908 * A101910 A101911 A101912


KEYWORD

easy,nonn


AUTHOR

Cino Hilliard, Jan 28 2005


STATUS

approved



