Riemann integral

The Riemann integral of $f$ equals $S$ if the following condition holds:

For all $\epsilon > 0$, there exists a $\delta$ such that for any tagged partition $x_0, \ldots , x_n$ and $t_0, \ldots , t_{n-1}$ whose mesh is less than $\delta$, we have
$$| \sum_{i=0}^{n-1} f(t_i) ( x_{i+1} - x_i) - S | < \epsilon$$

(Proof from first principle)

The function $f: [ a , b ] \rightarrow \mathbb{R}$ is Riemann integrable if and only if (it is bounded and the set of points at which it is discontinuous has Lebesgue measure zero).