A080356 Number of twin primes between n and 2n: a(n) = number of j in range n <= j <= 2*n such that j and j+2 are primes.

0, 1, 2, 1, 1, 1, 1, 1, 2, 2, 2, 1, 1, 1, 2, 2, 2, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 4, 4, 4, 4, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 6, 6, 6, 7, 7, 7, 6, 6, 6, 6

Offset: 1

Author

Labos Elemer, Feb 19 2003

Keywords

• nonn

Examples

			n=36: between 36 and 72 the primes p such that (p,p+2) is a pair of twin primes are {41,59,71}, so a(36)=3.
		

Links

• T. D. Noe, Table of n, a(n) for n=1..10000

Crossrefs

Cf. A001359, A071538.

Programs

Mathematica

Table[Count[Table[PrimeQ[j]&&PrimeQ[j+2], {j, n, 2*n}], True], {n, 1, 1000}]

Extensions

Definition clarified by N. J. A. Sloane, Aug 22 2021