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divisible by 11? (Posted on 2006-09-04) Difficulty: 3 of 5
I draw numbers 1 through k (k≤10) out of a hat ten times at random, replacing the numbers after drawing them. If I disregard the case where I draw "1" all ten times, explain why the number of possible sequences is divisible by 11. (Result by a calculator is insufficient because anyone can do that easily.)

Now if I change the number '10' to another integer n in the above paragraph, can I still have a similar result; i.e., the total possible number of configurations is divisible by n+1? Does this work for all integers n? If so, prove it; if not, find all integers n it works for.

No Solution Yet Submitted by Bon    
Rating: 3.5000 (2 votes)

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re(3): Replace '10' by n | Comment 12 of 17 |
(In reply to re(2): Replace '10' by n by Bractals)

n+1 must divide k^n - 1 for all k=1,2,...,n which includes n^n - 1. This is true if and only if n+1 is a prime. The "if" is Little Fermat. The "only if" is because if n=1 is not prime, there is a prime divisor p of n+1 in [1,n], and p^n - 1 = m(n+1) = mpq is impossible since it requires 1 to be a multiple of p.

  Posted by Richard on 2006-09-04 20:21:48

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