A046070 Second smallest m such that (2n-1)2^m-1 is prime, or -1 if no such value exists.

3, 1, 4, 5, 3, 26, 7, 2, 4, 3, 2, 6, 9, 2, 16, 5, 3, 6, 2553, 24, 10, 31, 2, 14, 5, 9, 6, 3, 2, 16, 5, 3, 6, 9, 4, 14, 11, 3, 4, 3, 5, 4, 11, 2, 8, 3, 4, 6, 9, 4, 18, 7, 3, 12, 149, 3, 14, 3, 2, 16, 3, 3, 4, 113, 3, 14, 11, 9, 18, 5, 2, 4, 13, 2, 16, 221, 4, 8, 5, 4, 6, 31, 3, 6, 5, 3, 4, 3

Offset

1,1

Comments

There exist odd integers 2k-1 such that (2k-1)2^n-1 is always composite.

References

Ribenboim, P. The New Book of Prime Number Records. New York: Springer-Verlag, pp. 357-359, 1996.

Links

Table of n, a(n) for n=1..88.

Eric Weisstein's World of Mathematics, Riesel Number.

Mathematica

max = 10000 (* this maximum value of m is sufficient up to n=168 *); a[n_] := Reap[ For[m = 0; cnt = 0, m <= max && cnt < 2, m++, If[m == max, Sow[-1], If[PrimeQ[(2*n - 1)*2^m - 1], cnt++; Sow[m]]]]][[2, 1]]; Table[a[n][[2]], {n, 1, 88}] (* Jean-François Alcover, Feb 28 2013 *)

Crossrefs

Cf. A046068, A046069.

Sequence in context: A324288 A302917 A069203 * A068399 A225407 A353292

Adjacent sequences: A046067 A046068 A046069 * A046071 A046072 A046073

Keyword

nonn

Author

Eric W. Weisstein

Status

approved