# Riemann sum

In mathematics, a Riemann sum is a method for approximating the values of integrals.

Let it be supposed there is a function f: DR where D, RR and that there is a closed interval I = [a,b] such that ID. If we have a finite set of points {x0, x1, x2, ... xn} such that a = x0 < x1 < x2 ... < xn = b, then this set creates a partition P = {[x0, x1), [x1, x2), ... [xn-1, xn]} of I.

If [itex]P[itex] is a partition with [itex]n \in \mathbb{N}[itex] elements of [itex]I[itex], then the Riemann sum of [itex]f[itex] over [itex]I[itex] with the partition [itex]P[itex] is defined as

[itex]S = \sum_{i=1}^{n} f(y_i)(x_{i}-x_{i-1})[itex]

where xi-1 ≤ yi ≤ xi. The choice of yi is arbitrary. If yi = xi-1 for all i, then S is called a left Riemann sum. If yi = xi, then S is called a right Riemann sum.

Suppose we have

[itex]S = \sum_{i=1}^{n} v_i(x_{i}-x_{i-1})[itex]

where vi is the supremum of f over [xi-1, xi]; then S is defined to be an upper Riemann sum. Similarly, if vi is the infimum of f over [xi-1, xi], then S is a lower Riemann sum.

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