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Old 2019-01-03, 00:15   #1
Jan 2019

1 Posts
Default Is there a way to check a certain exponent for whether or not (2^that exponent) - 1 is a prime?

I want to check whether or not a certain exponent will lead to a prime. When I started running GIMPS, I was automatically assigned an exponent. Do you think there is a way to change the program to check for a different exponent?
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Old 2019-01-03, 03:11   #2
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Nov 2006
Saskatchewan, Canada

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Stop Prime95 (or mprime if you are on Linux).
Edit worktodo.txt and change the exponent to the one you want to test.
Start Prime95.

2 things to consider first:
1. If someone is already testing that exponent you may get a "na" (No Assignement) error.

2. Most importantly the exponent you are testing must itself be a prime number for Prime95 to accept it.
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Old 2019-01-03, 03:20   #3
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Aug 2002
Yeehaw, FL

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Check here to make sure it has no known factors:

If the exponent is large, say above 200,000,000 expect a really long run time. Also consider doing a PRP test (better error checking) rather than an LL test.
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Old 2019-01-03, 04:11   #4
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Dec 2003

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For PRP test change worktodo.txt to:


But check first that no one has assigned that exponent, just change <exponent> to the actual number:<exponent>

If one LL test has already been done on it, and you want to double check it, do not do a PRP test, do another LL test:


Last fiddled with by ATH on 2019-01-03 at 04:12
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Old 2019-01-03, 06:52   #5
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Aug 2003
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Firstly, Welcome to GIMPS. We hope you have fun.

If you want to find a new world record prime, don't worry you are in the right place.

Note that the first number that you were assigned to check was almost certainly a double check. This helps us verify that no error overlooked a prime and checks your machine. Prime95 finds faults in machines all the time, even new ones. I had a brand new major brand computer fail testing right out of the box. I returned it for a replacement, that one passed. So, please consider letting the test run through to the end. You might get another double check on occasion to make sure your machine is still ok.

Right now all new first time checks being handed out are in the world record range. So, there is no need to make special requests.

If you are wanting to try to find the first 100 million digit prime, like George said it will take quite a while (several months). Let Prime95 check your machine with a double check first. Then set it to do the PRP test on it (hint look through the menus and settings). Also, many of the lowest numbers in the 100M digit range are assigned or have known factors. Check its status first. Also, there are ways to get an assignment like you want without doing the lookup work yourself (look at Manual assignments on the server.)

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