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A046069 Smallest m >= 0 such that (2n-1)2^m-1 is prime, or -1 if no such value exists. 9
2, 0, 2, 1, 1, 2, 3, 1, 2, 1, 1, 4, 3, 1, 4, 1, 2, 2, 1, 3, 2, 7, 1, 4, 1, 1, 2, 1, 1, 12, 3, 2, 4, 5, 1, 2, 7, 1, 2, 1, 3, 2, 5, 1, 4, 1, 3, 2, 1, 1, 10, 3, 2, 10, 9, 2, 8, 1, 1, 12, 1, 2, 2, 25, 1, 2, 3, 1, 2, 1, 1, 2, 5, 1, 4, 5, 3, 2, 1, 1, 2, 3, 2, 4, 1, 2, 2, 1, 1, 8, 3, 4, 2, 1, 3, 226, 3, 1, 2 (list; graph; refs; listen; history; text; internal format)
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

T. D. Noe, Table of n, a(n) for n=1..1000

Eric Weisstein's World of Mathematics, Riesel Number.

MATHEMATICA

max = 10^6; (* this maximum value of m is sufficient up to n=1000 *) a[1] = 2; a[2] = 0; a[n_] := For[m = 1, m <= max, m++, If[PrimeQ[(2*n - 1)*2^m - 1], Return[m]]] /. Null -> -1; Reap[ Do[ Print[ "a(", n, ") = ", a[n]]; Sow[a[n]], {n, 1, 100}]][[2, 1]] (* Jean-Fran├žois Alcover, Nov 15 2013 *)

CROSSREFS

Cf. A046067, A046070.

Bisection of A040081.

Sequence in context: A000586 A029399 A249338 * A055651 A175929 A079627

Adjacent sequences:  A046066 A046067 A046068 * A046070 A046071 A046072

KEYWORD

nonn

AUTHOR

Eric W. Weisstein

STATUS

approved

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Last modified June 24 07:19 EDT 2017. Contains 288697 sequences.