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

1,3

Links

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

Example

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.

Mathematica

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

Crossrefs

Cf. A001359, A071538.

Sequence in context: A138559 A073454 A124765 * A184167 A352085 A036541

Adjacent sequences: A080353 A080354 A080355 * A080357 A080358 A080359

Keyword

nonn

Author

Labos Elemer, Feb 19 2003

Extensions

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

Status

approved