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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
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
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
Sequence in context: A276863 A227321 A309555 * A179847 A035936 A006671
KEYWORD
nonn
AUTHOR
Pierre CAMI, Oct 07 2015
STATUS
approved