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This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

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

Original entry on oeis.org

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

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Author

Reinhard Zumkeller, Nov 13 2003

Keywords

Comments

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

Examples

			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.
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