3
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245 views

Prove: Number of Derangement is odd if and only if number of items is even .

let $D_n$ be a number of Derangement of n items . prove that $D_n$ is odd if and only if n is even . i was trying to use induction on the $!n=(n-1)(!(n-1)+!(n-2))$ recurrence relation but i cant ...
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194 views

discrete math party question

I have a discrete math question here but im not sure how to approach it with inclusion-exclusion. Can someone help me out? Consider a party of 60 persons. Assume that each person knows more than m ...