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 A126717 Least odd k such that k*2^n-1 is prime. 11
 3, 3, 1, 1, 3, 1, 3, 1, 5, 7, 5, 3, 5, 1, 5, 9, 17, 1, 3, 1, 17, 7, 33, 13, 39, 57, 11, 21, 27, 7, 213, 1, 5, 31, 3, 25, 17, 21, 3, 25, 107, 15, 33, 3, 35, 7, 23, 31, 5, 19, 11, 21, 65, 147, 5, 3, 33, 51, 77, 45, 17, 1, 53, 9, 3, 67, 63, 43, 63, 51, 27, 73, 5, 15, 21, 25, 3, 55, 47, 69 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS If a(n)=1 then n is a Mersenne exponent (A000043). - Pierre CAMI, Apr 22 2013 From Pierre CAMI, Apr 03 2017: (Start) Empirically, as N increases, (Sum_{n=1..N} a(n)) / (Sum_{n=1..N} n) tends to log(2); this is consistent with the prime number theorem as the probability that x*2^n - 1 is prime is ~ 1/(n*log(2)) if n is large enough. For n=1 to 10000, a(n)/n < 7.5. a(n)*2^n - 1 and a(n)*2^n + 1 are twin primes for n = 1, 2, 6, 18, 22, 63, 211, 282, 546, 726, 1032, 1156, 1321, 1553, 2821, 4901, 6634, 8335, 8529; corresponding values of a(n) are 3, 1, 3, 3, 33, 9, 9, 165, 297, 213, 177, 1035, 1065, 291, 6075, 2403, 2565, 4737, 3975, 459. (End) LINKS Pierre CAMI, Table of n, a(n) for n = 0..10000  (first 1000 terms from T. D. Noe) Ray Ballinger, Proth Search Page FORMULA a(n) << 19^n by Xylouris' improvement to Linnik's theorem. - Charles R Greathouse IV, Dec 10 2013 Conjecture: a(n) = O(n log n). - Thomas Ordowski, Oct 15 2014 EXAMPLE a(10)=5 because 5*2^10-1 is prime but 1*2^10-1 and 3*2^10-1 are not. MATHEMATICA f[n_] := Block[{k = 1}, While[ !PrimeQ[k*2^n - 1], k += 2]; k]; Table[f@n, {n, 0, 80}] (* Robert G. Wilson v, Feb 20 2007 *) PROG (PARI) a(n) = {my(k=1); while(!isprime(k*2^n - 1), k+=2); k}; \\ Indranil Ghosh, Apr 03 2017 (Python) from sympy import isprime def a(n): ....k=1 ....while True: ........if isprime(k*2**n - 1): return k ........k+=2 print [a(n) for n in xrange(101)] # Indranil Ghosh, Apr 03 2017 CROSSREFS Cf. A035050, A057778, A085427, A284631. Sequence in context: A214281 A125300 A303992 * A124039 A096433 A084101 Adjacent sequences:  A126714 A126715 A126716 * A126718 A126719 A126720 KEYWORD nonn AUTHOR Bernardo Boncompagni, Feb 13 2007 EXTENSIONS More terms from Robert G. Wilson v, Feb 20 2007 STATUS approved

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Last modified July 22 16:38 EDT 2019. Contains 325224 sequences. (Running on oeis4.)