Quote:
Originally Posted by storm5510
...I see are in the form of x*2^{p}1. Yes, this is an expansion of the standard Mersenne format. Why not be more creative?

Because when you add coefficient (your 'x'), the requirement for 'p' to be prime goes out of the window. 'p' can be any number. Easily can be an even number. just have a closer look at the UTM lists.
So don't use 'p' notation, and then you just fall through into the Riesel primes. Congrats for rediscovering them!