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

Author

Felix Fröhlich, Jun 18 2021

Keywords

• sign

Comments

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

Examples

			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.
		

Links

• Joe O, Project Description, Mersenne forum.
• Reggie, Welcome to the Sierpinski/Riesel Base 5 Project, PrimeGrid forum.
• Wikipedia, Riesel number

Crossrefs

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

Cf. A273987.

Programs

PARI

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