Let $\pi$ be a permutation of the integers $0$ through $2^n-1$ (the integers that can be represented in $n$ bits). For example, $\pi$ could be used as a substitution cipher in which each plaintext block is $n$ bits long. How many possible choices of permutation $\pi$ are there? Equivalently, how many permutations are there of the first $2^n$ non-negative integers?
closed as off-topic by fgrieu, DrLecter, rath, figlesquidge, Maeher Jan 28 at 10:35
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