OFFSET
1,11
COMMENTS
31532322469 (A273987(3)/2) is the smallest n such that a(n) = -1.
LINKS
Wikipedia, Riesel number
EXAMPLE
For n = 11: (2*11)*3^k-1 is prime for k = 2, with 2 being the smallest such k, so a(11) = 2.
PROG
(PARI) a(n) = for(k=0, oo, if(ispseudoprime((2*n)*3^k-1), return(k)))
CROSSREFS
KEYWORD
sign
AUTHOR
Felix Fröhlich, May 04 2021
STATUS
approved