OFFSET
1,12
COMMENTS
a(159986/2) = a(79993) = -1.
LINKS
Joe O, Project Description, Mersenne forum.
Reggie, Welcome to the Sierpinski/Riesel Base 5 Project, PrimeGrid forum.
Wikipedia, Sierpiński number
EXAMPLE
For n = 17: 34*5^k + 1 is composite for k = 0, 1, 2, 3, 4, 5, 6, 7 and prime for k = 8. Since 8 is the smallest such k, a(17) = 8.
PROG
(PARI) a(n) = for(k=0, oo, if(ispseudoprime((2*n)*5^k+1), return(k)))
CROSSREFS
KEYWORD
sign
AUTHOR
Felix Fröhlich, Jun 24 2021
STATUS
approved