A089881 Number of primes <= n that are members of twin prime pairs.

0, 0, 0, 0, 2, 2, 3, 3, 3, 3, 3, 3, 5, 5, 5, 5, 5, 5, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15

Offset

1,5

Comments

a(n) = if n<3 then 0 else (if n<7 then A071538(n-2)*2 else A071538(n-2)*2-1).

Links

Table of n, a(n) for n=1..82.

Example

a(30) = 7 = #{3,5,7,11,13,17,19}, as (3,5), (5,7), (11,13) and (17,19) are all twin prime pairs with members not greater than 30,
a(31) = 9 = #{3,5,7,11,13,17,19,29,31} with two more members for the next twin prime pair (29,31).

Crossrefs

Cf. A001359, A006512, A089882.

Sequence in context: A147954 A105047 A331135 * A251547 A084320 A161358

Adjacent sequences: A089878 A089879 A089880 * A089882 A089883 A089884

Keyword

nonn

Author

Reinhard Zumkeller, Nov 13 2003

Status

approved