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A104275
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Numbers n such that 2n-1 is not prime.
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6
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1, 5, 8, 11, 13, 14, 17, 18, 20, 23, 25, 26, 28, 29, 32, 33, 35, 38, 39, 41, 43, 44, 46, 47, 48, 50, 53, 56, 58, 59, 60, 61, 62, 63, 65, 67, 68, 71, 72, 73, 74, 77, 78, 80, 81, 83, 85, 86, 88, 89, 92, 93, 94, 95, 98, 101, 102, 103, 104, 105, 107, 108, 109, 110, 111, 113
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| Same as A053726 except for the first term of this sequence; also a(n) = A047845(n-1) + 1
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FORMULA
| a(n)=sum_{x=1..nth odd nonprime}-(-1)^x*x [From Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Jul 14 2009]
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EXAMPLE
| a(1)=1 because 2*1-1=1, not prime.
a(2)=5 because 2*5-1=9, not prime (2, 3 and 4 give 3, 5 and 7 which are primes)
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MATHEMATICA
| Select[ Range[115], !PrimeQ[2# - 1] &] (from Robert G. Wilson v (rgwv(AT)rgwv.com), Apr 18 2005)
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PROG
| (MAGMA) [n: n in [1..220]| not IsPrime(2*n-1)] [From Vincenzo Librandi, Jan 28 2011]
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CROSSREFS
| Cf. A005097, A047845, A006254, A053726.
Sequence in context: A191233 A189577 A047700 * A053726 A173977 A161537
Adjacent sequences: A104272 A104273 A104274 * A104276 A104277 A104278
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KEYWORD
| easy,nonn
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AUTHOR
| Alexandre Wajnberg (alexandre.wajnberg(AT)ulb.ac.be), Apr 17 2005
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EXTENSIONS
| More terms from Robert G. Wilson v (rgwv(AT)rgwv.com), Apr 18 2005
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