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Topic: Dice sum divisibility (Read 2998 times) 

Eigenray
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Dice sum divisibility
« on: Aug 7^{th}, 2008, 9:41am » 
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Let S_{n} denote the sum of n standard dice. In increasing order of difficulty: 0) If m6, show that for all n, S_{n} is uniformly distributed mod m. 1) Find all m,n such that S_{n} is uniformly distributed mod m. 2) For each m 12, find all n such that the probability that m divides S_{n} is 1/m. 3) Show that for any m > 12, there are at most finitely many n for which P( m  S_{n} ) = 1/m. 4) Are there any pairs m,n, m > 12, for which P( m  S_{n} ) = 1/m?

« Last Edit: Aug 30^{th}, 2008, 12:12pm by Eigenray » 
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l4z3r
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Re: Dice sum divisibility
« Reply #1 on: Aug 30^{th}, 2008, 5:44am » 
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on Aug 7^{th}, 2008, 9:41am, Eigenray wrote: S_{n} is uniformly distributed mod m. 
 i dont get this. uniformly distributed?


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towr
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Re: Dice sum divisibility
« Reply #2 on: Aug 30^{th}, 2008, 11:37am » 
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on Aug 30^{th}, 2008, 5:44am, l4z3r wrote:i dont get this. uniformly distributed? 
 It means any residue modulo m is equally probable. So take, for example: n=2 m=6 We can get sums 2 (x1), 3 (x2), 4 (x3), 5 (x4), 6 (x5), 7 (x6), 8 (x5), 9 (x4), 10 (x3), 11 (x2), 12 (x1) modulo 6, we get 2 = 2 (mod 6) x1 3 = 3 (mod 6) x2 4 = 4 (mod 6) x3 5 = 5 (mod 6) x4 6 = 0 (mod 6) x5 7 = 1 (mod 6) x6 8 = 2 (mod 6) x5 9 = 3 (mod 6) x4 10 = 4 (mod 6) x3 11 = 5 (mod 6) x2 12 = 0 (mod 6) x1 So we have 6 out of 36 of each of 0..5 (mod 6), they are all equally probable residues.


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Eigenray
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Re: Dice sum divisibility
« Reply #3 on: Aug 30^{th}, 2008, 12:14pm » 
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How about starting with something easier then: 0) If m6, show that for all n, S_{n} is uniformly distributed mod m. That is, S_{1} is uniformly distributed mod m if and only if (for all n, S_{n} is uniformly distributed mod m). Hint for the rest: generating functions and roots of unity.


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Michael Dagg
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Re: Dice sum divisibility
« Reply #4 on: Aug 31^{st}, 2008, 5:50pm » 
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Nice problem set. In Towr's example, the probability mass function of the sum is a triangle, so the sum of the slopes at the two sides is a constant. This does not seem to be true for n>2 . (Generalizing, it seems to work also with n=2 and any m , if you use msided rather than 6sided dice.)


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towr
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Re: Dice sum divisibility
« Reply #5 on: Sep 1^{st}, 2008, 12:34am » 
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Mod 6, any extra die just adds a random "shift", uniformly. So it doesn't change the distribution mod 6. And if the distribution is uniform modulo k, then it's uniform modulo m for any mk, because you take an equal number of groups together (k/m).


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william wu
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Re: Dice sum divisibility
« Reply #6 on: Oct 17^{th}, 2008, 9:46am » 
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Problem 0) Here's a long way of saying the same thing towr said ... just my own way of seeing it. Have not got to the other problems yet. To show that S_n is uniformly distributed mod m, it suffices to show that S_2 is uniformly distributed mod m. We can then deduce a uniform distribution for any larger sums by induction. Let X be a random variable describing the outcome of the first roll of the die, and Y denote the outcome of the second roll. Then S_2 = X+Y. Now imagine making a 6x6 matrix that enumerates all the possible ways sums can be made from X and Y. The rows are decorated with the possible values of X, and the columns are decorates with the possible values of Y. Then for i {1,2,3,4,5,6} and j {1,2,3,4,5,6}, the ijth entry of the matrix contains i + j: We can make the matrix is Toeplitz by reversing the ordering of the columns, so that instead of the jth column representing Y=j, it will now represent Y = 6j+1. Then Now, the ijth entry contains (6  j + 1) + i = (ij) + (6+1), a function of (ij), which indicates that the matrix is Toeplitz. Lastly, let m be any factor of 6, and take residues of our Toeplitz matrix mod m. For example, if m=6, the matrix of residues is The ijth entry is now ( (ij) + (6+1) ) mod 6 = ((ij) + 1) mod 6 a function of (ij) mod 6, which makes this a circulant matrix. Thus, to assure that every entry in a circulant matrix appears the same number of times, it suffices to show that every entry in the first column appears an equal number of times. Setting j=1, we get that the ith entry of the first column is: i mod 6 : i {1,2,3,4,5,6} and thus all numbers in {0,1,2,3,4,5} appear exactly once, proving that we have an uniform distribution mod 6. Similarly, if m is any factor of 6, and we take residues mod m, then the ith entry of the first column is i mod m : i {1,2,3,4,5,6} and since m  6, every possible residue will occur the same number of times; namely, (6/m) times. To show that the result holds as well for a die with k sides, just replace all instances of 6 above with k.

« Last Edit: Oct 17^{th}, 2008, 9:50am by william wu » 
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