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 A046068 Second smallest m such that (2n-1)2^m+1 is prime, or -1 if no such value exists. 4
 1, 2, 3, 4, 2, 3, 8, 2, 15, 10, 4, 9, 4, 4, 3, 60, 6, 3, 4, 2, 11, 6, 9, 1483, 6, 3, 5, 8, 3, 11, 12, 4, 3, 6, 2, 5, 6, 3, 7, 10, 4, 5, 6, 6, 7, 168, 4, 3, 4, 2, 9, 18, 2, 7, 14, 4, 5, 12, 4, 3, 12, 8, 5, 12, 5, 3, 6, 2, 27, 14, 3, 77, 16, 11, 7, 20, 2, 7, 12, 7, 5, 4, 2, 103, 14, 9, 13, 4 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 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 Eric Weisstein's World of Mathematics, Riesel Number. Eric Weisstein's World of Mathematics, Sierpiński Number of the Second Kind MATHEMATICA max = 10000 (* this maximum value of m is sufficient up to n=191 *); a[n_] := Reap[ For[m = 1; 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]]; a[1] = {0, 1}; Table[a[n][[2]], {n, 1, 88}] (* Jean-François Alcover, Feb 27 2013 *) CROSSREFS Cf. A046067, A046070. Sequence in context: A304112 A293446 A256443 * A166281 A330954 A107468 Adjacent sequences:  A046065 A046066 A046067 * A046069 A046070 A046071 KEYWORD sign AUTHOR STATUS approved

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Last modified August 1 07:45 EDT 2021. Contains 346384 sequences. (Running on oeis4.)