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A113609
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Number of prime powers q<=n such that also q+2 is a prime power.
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0
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1, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 7, 7, 7, 7, 8, 8, 8, 8, 8, 8, 9, 9, 10, 10, 11, 11, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 13, 13, 13, 13, 13, 13, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 16, 16, 16, 16, 16, 16, 16
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| a(n) > A071538(n);
(25,27) is the samallest pair of prime powers (q,q+2) such that both q and q+2 are not primes, conjecture: there are more (but not < 10^6).
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LINKS
| Eric Weisstein's World of Mathematics, Prime Power
Index entries for sequences related to numbers of primes in various ranges
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EXAMPLE
| a(10) = #{(p^0,3),(2,2^2),(3,5),(5,7),(7,3^2),(3^2,11)} = 6.
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CROSSREFS
| Cf. A000961.
Sequence in context: A137687 A024745 A030581 * A067086 A005410 A120835
Adjacent sequences: A113606 A113607 A113608 * A113610 A113611 A113612
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KEYWORD
| nonn
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AUTHOR
| Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Jan 14 2006
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