How Many Unique 5 White and 5 Black Bead Necklaces Can Be Made?

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SUMMARY

The problem of constructing unique necklaces with 5 white beads and 5 black beads is a circular permutation challenge that requires accounting for rotational symmetry and reflections. The initial calculation of 10!/5!5! yields 252 arrangements, but this does not consider the symmetries involved. By fixing a pair of adjacent beads and analyzing the symmetries, the final count of unique necklaces is determined to be 16. This solution emphasizes the importance of breaking down the problem into cases based on symmetry counts.

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Homework Statement



How many necklace with 5 white beads and 5 black beads can be constructed?


Homework Equations



Circular Permutation problem

The Attempt at a Solution

]

I did 10!/5!5!=252

but from there I didn't get anywhere.

I know this includes repeats from rotational symmetry and reflections. but i am not really sure how to get rid of these.

i try dividing by 10 but it gives 25.2, which does not make sense to me.
 
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There is no simple way to analyse this sort of problem. You need to break it down into cases according to the symmetries.
You know there must somewhere be a black and a white adjacent, so you could fix on such a pair. That gets you down to 8-choose-4 immediately. Then it's a matter of removing duplicates.
 
Thinking some more about this... consider how many symmetries any given pattern might have. If only one (i.e. the identity) how many times will the given unique pattern be counted in your 252? What if two symmetries in the group? Etc. Then it's a matter of figuring how many patterns have each of the symmetry counts.
Fwiw, I make the final answer 16.
 

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