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Probability of having a log(n) length monotone subsequence in a random permutation of {1,...,n}

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Problem Detail: 

How can I compute the probability of having a $\log(n)$ length monotone consecutive subsequence in a random permutation of $\{1,...,n\}$.

I wish to upperbound it with $1/n$.

Asked By : JonyWalk
Answered By : Yuval Filmus

You can upper bound this probability as follows. The relative order within a consecutive subsequence of a random permutation is just another random permutation, so the probability that a specific consecutive subsequence of length $\log n$ is monotone is exactly $2/(\log n )!$. Since there are at most $n$ of this (actually, exactly $n-\log n+1$), the probability is at most $$ \frac{2n}{(\log n)!} \sim \frac{2n}{\sqrt{2\pi\log n}(\log n/e)^{\log n}} = o\left(\frac{1}{n}\right). $$

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