Suppose we have an array of $n$ integers. Suppose that we pick one of these elements uniformly at random and call it $x$. Suppose that $\log n$ elements are also sampled (uniformly at random) from the $n$-size array and we call $A$ this sampled array. What is the probability that $x$ lies in the middle third of the sorted version of the array $A$?

Initially I thought that this probability was $1/3$, but I think it's more complex than that. I believe that this probability should be $$ \text{Pr}[x \geq z]^{\frac{\log n}{3}}\cdot \text{Pr}[x \leq z]^{\frac{\log n}{3}},$$ where $z$ is an element sampled from the original size $n$ array. I don't know if this is correct or how to compute these probabilities.


1 Answer 1


$$ \def\P{{\mathbb P}} \def\E{{\mathbb E}} \def\I{{\mathbb I}} $$

The probability is approximately $\frac 13$. "Approximately" depends on $n$, your definition of "third" (i.e. what to do if $3$ doesn't divide $1 + \log n$), and on whether you sample without or with replacement (and in this case, how you handle multiple appearances of the same element). To give an extreme example, if $\log n = 0$, then the probability is either $0$ or $1$ depending on how you define "in the middle third".

Intuitively. Your process is equivalent to the following: sample $\log n + 1$ elements ($A$ and $x$), sort them, choose the random element among them, and ask what's the probability that its position is in the middle third. Clearly, the answer is approximately $1/3$ (the exact answer depends on the clarifications above).

Formally. Let's introduce the following notation: $A = (a_1, \ldots, a_k)$ and $a_0 = x$. I assume that $3$ divides $k+1$. I say that the element is in the middle third if the number of elements that are less than it is in range $[\frac{k+1}{3}, \frac{2k - 1}{3}]$. Let $C_i$ be an event that $a_i$ is in the middle third.

The problem asks about $\P[C_0]$. Since $a_0, \ldots, a_k$ are sampled from the same distribution, by symmetry we have $\P[C_0] = \cdots = \P[C_k]$. Let $C_i$ be the random variable denoting the number of elements less than $x$. Clearly, for any specific sample, exactly $\frac 13$ of $C_0, \ldots, C_k$ hold: namely, if $\I$ is the indicator function, then $\sum_{i=0}^k \I[C_i] = \frac{k+1}{3}$. Using $\P[C_i] = \E[\I[C_i]]$, we have: \begin{align*} \P_{a_0,\ldots,a_k}[C_0(a_0,\ldots,a_k)] &= \frac{1}{k+1}\sum_{i=0}^k \P_{a_0,\ldots,a_k}[C_i(a_0,\ldots,a_k)] \\ &= \frac{1}{k+1}\sum_{i=0}^k \E_{a_0,\ldots, a_k} \I[C_i(a_0,\ldots, a_k)] \\ &= \frac{1}{k+1} \E_{a_0,\ldots, a_k} \sum_{i=0}^k \I[C_i(a_0,\ldots, a_k)] \\ &= \frac{1}{k+1} \E_{a_0,\ldots, a_k} \frac{k+1}{3} \\ &= \frac 13 \end{align*}


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