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(x!+1)/(x+1) = prime number (Posted on 2010-07-30) Difficulty: 3 of 5
For a positive integer x drawn at random between 1 and 20000 inclusively, determine the probability that (x!+1)/(x+1) is a prime number.

No Solution Yet Submitted by K Sengupta    
Rating: 2.3333 (3 votes)

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re: mathematica solutions (so far) | Comment 6 of 9 |
(In reply to mathematica solutions (so far) by Daniel)

Nice find Daniel.

I am giving up on a (quick) solution to this, as rather than find a program already made to determine if a number is a Strong Probable Prime, I got sidetracked and began trying to code my own. I had to learn about modular exponentiation, and brush up on my Python skills just to code a very rough version of the algorithm on Wikipedia. 


I learned that Python 3 no longer likes large integers, and had to write another function, just to break down my values into chunks that it could work with. All of this "wasted" time for a program that can't compare to even the most horribly coded (but functional) version of the algorithm when done in C++. Is it time to relearn the language? I think so!

So, thanks to this problem's ability to get me thinking (and learning), I decided to rate it very highly (perhaps influenced further by my love of prime numbers). Hopefully we're able to come to a conclusion as to what the final answer is sometime in the near future.

  Posted by Justin on 2010-07-31 03:57:51
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