# Monte-Carlo Integration: the Error Function

#### Question

The error function, $\mathrm{erf}(x)$ is defined as $$\mathrm{erf}(x) = \frac{2}{\sqrt{\pi}}\int_0^x\mathrm{e}^{-t^2}\;\mathrm{d}t.$$ The integral cannot be evaluated in closed form, but many numerical approximations and converging series for $\mathrm{erf}(x)$ are known. Python's math module provides the method erf() for its evaluation.

One approximate method for evaluating the error function is Monte Carlo integration. $f(t) = \mathrm{e}^{-t^2}$ is a monotonically decreasing function of $x$, and $\mathrm{e}^{0} = 1$ so consider the rectangular region of the $tf$-plane bounded by $0 \le t \le x$ and $f(x) \le f \le 1$. The area under curve $f(t)$ can be approximated by picking a large number of random points inside this rectangle, and counting how many of them lie under the curve as a proportion of the total number picked.

Write a program to estimate the value of $\mathrm{erf}(\frac{1}{2})$ and compare it with the result of the builtin math.erf(0.5)