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 A104279 Numbers n such that 2n+1 is prime and 2n-1 is not prime. 2
 1, 5, 8, 11, 14, 18, 20, 23, 26, 29, 33, 35, 39, 41, 44, 48, 50, 53, 56, 63, 65, 68, 74, 78, 81, 83, 86, 89, 95, 98, 105, 111, 113, 116, 119, 125, 128, 131, 134, 138, 140, 146, 153, 155, 158, 165, 168, 173, 176, 179, 183, 186, 189, 191, 194, 198, 200, 204, 209, 215 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS EXAMPLE a(1)=1 because 3 is prime and 1 is not prime. a(2)=5 because 11 is prime and 9 is not prime. MATHEMATICA Select[ Range[218], PrimeQ[2# + 1] && !PrimeQ[2# - 1] &] (* Robert G. Wilson v, Apr 18 2005 *) PROG (MAGMA) [n: n in [1..220]| not IsPrime(2*n-1) and IsPrime(2*n+1)] // Vincenzo Librandi, Jan 28 2011 (PARI) is(n)=isprime(2*n+1) && !isprime(2*n-1) \\ Charles R Greathouse IV, Jun 13 2017 CROSSREFS Cf. A040040 and others. Sequence in context: A314388 A022798 A022808 * A184919 A063247 A184928 Adjacent sequences:  A104276 A104277 A104278 * A104280 A104281 A104282 KEYWORD easy,nonn AUTHOR Alexandre Wajnberg, Apr 17 2005 EXTENSIONS More terms from Robert G. Wilson v, Apr 18 2005 STATUS approved

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Last modified January 21 05:39 EST 2020. Contains 331104 sequences. (Running on oeis4.)