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Old 2007-09-04, 15:15   #2
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"Tim Sorbera"
Aug 2006
San Antonio, TX USA

2×3×23×31 Posts

Originally Posted by jasong View Post
Is the following true? Or, at least, assumed to be true?

odds of a random n yielding a Mersenne prime:


I had a hell of a time coming up with the qualifier(the second line). Corrections are appreciated and encouraged.

Btw, I put this in Information and Answers because it's fairly simple math, and I feel that a lot of people that might not normally visit the Math forum might be interested in this, since it has to do with Mersenne numbers.

Edit: Another question: Assuming GIMPS throughput increases at a sustained rate, basically a parabolical curve, will our speed at finding primes be likely to increase, decrease, or stay exactly the same? (Maybe this one should go in Math, and the answer should come back here when it's decided upon)
Originally Posted by
What are the chances that the Lucas-Lehmer test will find a new Mersenne prime number? A simple approach is to repeatedly apply the observation that the chance of finding a factor between 2X and 2X+1 is about 1/x. For example, you are testing 210000139-1 for which trial factoring has proved there are no factors less than 264. The chance that it is prime is the chance of no 65-bit factor * chance of no 66 bit factor * ... * chance of no 5000070 bit factor. That is:
64 65 5000069
-- * -- * ... * -------
65 66 5000070
This simplifies to 64 / 5000070 or 1 in 78126. This simple approach isn't quite right. It would give a formula of how_far_factored divided by (exponent divided by 2). However, more rigorous work has shown the formula to be (how_far_factored-1) / (exponent times Euler's constant (0.577...)). In this case, 1 in 91623. Even these more rigourous formulas are unproven.
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