Quote:
Originally Posted by 3.14159
You mean, a prime number of the form k * 2^{n}  1? It should be equally difficult as a Proth search.

Oh please, don't edit your posts while others want to response to them!
Your original question: No, Riesel numbers are known and the Riesel problem want to find the smallest of them (k=509203 seems the candidate but not proven yet).
Riesel primes are so called, because H.Riesel listed k*2^n1 for some small kvalues and small nvalues first (in the 1950's if I'm right).
And yes: The difficulty for testing Proth or Riesel primes are the same.