A345403 Riesel problem in base 5: a(n) is the smallest k >= 0 such that (2*n)*5^k - 1 is prime, or -1 if no such k exists.

4, 0, 0, 0, 3, 0, 0, 1, 0, 0, 1, 0, 4, 1, 0, 0, 163, 1, 0, 1, 0, 0, 1, 0, 2, 5, 0, 2, 7, 0, 0, 5, 5, 0, 1, 0, 0, 1, 1, 0, 1, 0, 2058, 1, 0, 26, 5, 1, 0, 1, 0, 0, 3, 0, 0, 3, 0, 32, 17, 1, 2, 1, 3, 0, 3, 0, 8, 21, 0, 0, 1, 1, 4, 1, 0, 0, 1, 4, 0, 7, 1, 0, 1, 0

Offset

1,1

Comments

a(346802/2) = a(173401) = -1 (see A273987).

Links

Table of n, a(n) for n=1..84.

Joe O, Project Description, Mersenne forum.

Reggie, Welcome to the Sierpinski/Riesel Base 5 Project, PrimeGrid forum.

Wikipedia, Riesel number

Example

For n = 5: 10*5^k - 1 is composite for k = 0, 1, 2 and prime for k = 3. Since 3 is the smallest such k, a(5) = 3.

Prog

(PARI) a(n) = for(k=0, oo, if(ispseudoprime((2*n)*5^k-1), return(k)))

Crossrefs

Cf. A040081 (base 2), A343914 (base 3), A250205 (base 6).

Cf. A273987.

Sequence in context: A152898 A368661 A369009 * A210252 A028719 A028662

Adjacent sequences: A345400 A345401 A345402 * A345404 A345405 A345406

Keyword

sign

Author

Felix Fröhlich, Jun 18 2021

Status

approved