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Suppose I have a set $S$ with $n$ items, and I create the ordering (e.g. $\{a, b, c, d\}$) using every element of S exactly one. How many orderings could I create with these same items such that:

-All elements of S are in the new ordering

-Each element occurs only once.

-Element $i$ of the first ordering does not match element $i$ of the second ordering

(e.g. so if the first element is a in the first ordering, so the first element in the second ordering can't be a).

I think maybe a recursive/inductive approach could be useful, but I suspect there might be a method that doesn't require this and instead simplifies with combinatorics or something.

cmitch
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  • Recursive sounds good. Why don't you solve in whatever way you can, and once you find the answer try and think of simplifcations? – user619894 Oct 09 '21 at 19:23
  • Well recursive would end up prob with a summation of combinatorics, and simplifying a summation of combinatorics is something I am not skilled enough to do beyond knowing a situation enough to logic out n choose k situations, which I'm not fully sure of here – cmitch Oct 09 '21 at 20:16
  • Also to be clear this isn't like a hw question or anything haha, I've just been thinking of this and having a lot of trouble wrapping my brain around it – cmitch Oct 09 '21 at 20:17

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