Thread: Riesel primes
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Old 2009-07-18, 16:49   #9
Mini-Geek
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"Tim Sorbera"
Aug 2006
San Antonio, TX USA

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Remember that the restriction is 2^n>k, not n>k, so e.g. n=4 k=15 is allowed (2^4=16, 16>15) 15*2^4-1=239 which is prime. www.rieselprime.de lists primes when 2^n<k, even though these aren't technically Riesel numbers.
Quote:
Originally Posted by Primeinator View Post
Finally, n = k is also a possibility. 3*2^3 -1 = 23 = ! prime.
This might interest you: n*2^n+1 is a Cullen Number. (essentially a Proth number with k=n and no 2^n>k restriction; the Proth side equivalent of a Woodall number)
Quote:
Originally Posted by Primeinator View Post
Apparently, all three scenarios are possible?
Apparently.

So...
When k*2^n+1 is prime, k*2^n+1 is called a Proth prime
When k*2^n-1 is prime, k*2^n-1 is called a Riesel prime
When k*2^n+1 is composite for every n with this specific k, k is called a Sierpinski number
When k*2^n-1 is composite for every n with this specific k, k is called a Riesel number
When k*2^n+1 with odd k, positive integer n, and 2^n>k, k*2^n+1 is called a Proth number
When k*2^n-1 with odd k, positive integer n, and 2^n>k, k*2^n-1 is called ...what? (we're referring to it as Riesel number here, but that's technically incorrect since that refers to the equiv. of a Sierpinski number)

or in text: "Riesel number" technically refers to a k such that all k*2^n-1 are composite, and "Riesel prime" refers to primes of the form k*2^n-1, right? Is there any name for numbers of the form k*2^n-1, analogous to "Proth number" for numbers of the form k*2^n+1? I know there is rarely confusion, at least in projects that aren't searching for Riesel numbers, but it is still an incorrect and vague reference.
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