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 A262994 Smallest number k>2 such that k*2^n-1 is a prime number. 2
 3, 3, 3, 3, 4, 3, 3, 5, 7, 5, 3, 5, 9, 5, 4, 8, 4, 3, 28, 14, 7, 26, 13, 39, 22, 11, 16, 8, 4, 20, 10, 5, 6, 3, 24, 12, 6, 3, 25, 24, 12, 6, 3, 14, 7, 20, 10, 5, 19, 11, 21, 20, 10, 5, 3, 32, 16, 8, 4, 17, 24, 12, 6, 3, 67, 63, 43, 63, 40, 20, 10, 5, 15, 12, 6, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS If k=2^j then n+j is a Mersenne exponent. a(n)=3 if and only if 3*2^n-1 is a prime; that is, n belongs to A002235. - Altug Alkan, Oct 08 2015 LINKS Pierre CAMI, Table of n, a(n) for n = 1..10000 EXAMPLE 3*2^1-1=5 prime so a(1)=3; 3*2^2-1=11 prime so a(2)=3; 3*2^3-1=23 prime so a(3)=3. MATHEMATICA a[n_] := For[k = 3, True, k++, If[PrimeQ[k*2^n - 1], Return[k]]]; Table[a[n], {n, 1, 100}] (* Jean-François Alcover, Oct 07 2015 *) PROG (PARI) a(n) = {k=3; while (! isprime(k*2^n-1), k++); k; } \\ Michel Marcus, Oct 08 2015 CROSSREFS Cf. A247202, A002235. Sequence in context: A276863 A227321 A309555 * A179847 A035936 A006671 Adjacent sequences:  A262991 A262992 A262993 * A262995 A262996 A262997 KEYWORD nonn AUTHOR Pierre CAMI, Oct 07 2015 STATUS approved

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Last modified January 16 05:22 EST 2022. Contains 350374 sequences. (Running on oeis4.)