# Markov’s Inequality

Markov’s inequality provides a bound on the upper-tail probability of non-negative random variables. The bound is expressed with the expectation of the random variable.

[Markov's Inequality]
Let $$X$$ be a non-negative random variable. Then, for any $$\lambda > 0$$,
$Pr[ X \geq \lambda ] \leq \frac{E[X]}{\lambda}.$
Equivalently, for any $$k > 1$$,
$Pr[ X \geq k E[X] ] \leq \frac{1 }{k}.$

Proof.
Define a random variable $$Y$$ such that
$Y = \begin{cases} \lambda & X\geq \lambda, \\ 0 & X< \lambda. \\ \end{cases}$
Then $$E[X] \geq E[Y] \geq \lambda\cdot Pr[X\geq \lambda]$$.

Markov’s inequality only bounds the upper-tail probability. The following is a similar bound on the lower-tail probability if the random variable is bounded above.

[Lemma]
Let $$X$$ be a random variable over $$[0,1]$$. Then, for any positive $$k < 1$$,
$Pr[ X < k E[X] ] \leq \frac{1 – E[X]}{1 – k E[X]}.$

The following are some applications of Markov’s inequality.

[Borel-Cantelli Lemma]
Let $$E_1, E_2, \ldots$$ be a sequence of events such that $$\sum_n Pr[E_n] <\infty$$. Then the probability that no more than $$k$$ of the events happen is at least
$1 - \frac{ \sum_{n} Pr[E_n] }{ M }.$
This implies that, with probability 1, at most finitely many events happen.

Proof.
For each event $$E_i$$, define binary random variable $$X_i$$ such that $$X_i = 1$$ if $$E_i$$ hold, and $$X_i = 0$$ otherwise. Then $$E[X_i] = Pr[E_i]$$, and, by linearity of expectation,
$E[\sum_i X_i ] = \sum_i E[ X_i ] = \sum_i Pr[ E_i ].$
The result follows by applying Markov’s inequality on $$\sum_i X_i$$.

[Chebyshev's Inequality]
Let $$X$$ be a random variable. Then, for any $$\lambda > 0$$,
$Pr[ |X-\mathbb{E}(X)| \geq \lambda ] \leq \frac{ Var[X] }{\lambda^2}.$
Equivalently, for any $$k > 1$$,
$Pr[ |X-\mathbb{E}(X)| \geq k Var[X] ] \leq \frac{1}{k^2}.$

Chebyshev’s inequality follows by applying Markov’s inequality on the random variable $$(X – E[X])^2$$, and the fact that $$Var[X] = E [ (X - E[X])^2 ]$$.