# Homework 6 Optional Problems

Recall that a set \(H\) of hash functions (mapping the elements of a universe \(U\) to the buckets \(\{0,1,2,...,n - 1\})\) is universal if for every distinct \(x, y \in U\), the probability \(Prob[h(x) = h(y)]\) that \(x\) and \(y\) collide, assuming that the hash function \(h\) is chosen uniformly at random from \(H\), is at most \(\frac{1}{n}\). In this problem you will prove that a collision probability of \(\frac{1}{n}\) is essentially the best possible. Precisely, suppose that \(H\) is a family of hash functions mapping \(U\) to \(\{0,1,2,...,n - 1\}\), as above. Show that there must be a pair \(x, y \in U\) of distinct elements such that, if \(h\) is chosen uniformly at random from \(H\), then \(Prob[h(x) = h(y)] \geq \frac{1}{n} - \frac{1}{\lvert U \rvert}\)

**ANSWER:** Let \(n_i\) be the size of the set \(\{x: x \in U, h(x) = i\}\), for \(i \in \{0,1,2,...,n - 1\}\). That is, \(h_1\) maps inputs to bucket 1, \(h_2\) maps inputs to bucket 2, and so on.

- Let us fix a hash function \(h \in H\).
- Let \(N = \lvert U \rvert\).
- Given \(n\) = number of buckets.

Then, the number of distinct \(x_1, x_2 \in U\) that collide in bucket 1 is \(\binom{n_i}{2}\). The number of distinct \(x_1, x_2 \in U\) that collide in all buckets is, therefore, \(\sum_{i=1}^{n} \binom{n_i}{2}\) (call this expression \(L_h\)). Note that \(n_i \geq 0 \text{ and } \sum_{i=1}^{n} n_i = N\). We are interested in lower-bounding the expression \(L_h\).

\[\begin{equation*} \begin{aligned} L_h & \geq \sum_{i=1}^{n} \frac{n_i(n_i - 1)}{2} \\ & = \sum_{i=1}^{n} \frac{n_i^2}{2} - \sum_{i=1}^{n} \frac{n_i}{2} \\ & = \sum_{i=1}^{n} \frac{n_i^2}{2} - \frac{N}{2} \end{aligned} \end{equation*}\]It can be shown that the summation is minimized when all \(n_i\) are equal, that is, when \(n_i = \frac{N}{n}\) ^{1} ^{2}. Therefore, we get:

Suppose \(H = \{h_1,h_2,...,h_k\} \text{ and } \vert H \rvert = K\). Let us define \(P\) be the set of all distinct \(x_1, x_2 \in U\). Note that \(\lvert P \rvert = \frac{N(N-1)}{2}\). Let \(X_i\) be an indicator random variable, such that, given a hash function \(h_i \in H\) and distinct \(x_1, x_2 \in U\):

\[X_i = \begin{cases} 1 \text{ if } h_i(x_1) = h_i(x_2) = i \\ 0 \text{ otherwise } \end{cases}\]Taking expectation of \(X_i\):

\[\begin{equation*} \begin{aligned} E[X] & = \sum_{i=1}^{K}\sum_{x,y \in U} Pr[h_i(x_1) = h_i(x_2)] \\ & = \sum_{x,y \in U} \frac{1}{K}\sum_{i=1}^{K} L_h \\ & = \frac{1}{\lvert P \rvert} \cdot L_h \\ & = \frac{2}{N(N-1)} \cdot \frac{N}{2}(\frac{N}{n} - 1) \\ & \geq \frac{\frac{N}{n} - 1}{N} \\ & = \frac{1}{n} - \frac{1}{N} \end{aligned} \end{equation*}\]By the Pigeon hole Principle, we get: There exists \(x, y \in U\) such that the above inequality holds.

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