Definition:Lower Sum

Definition
Let $\left[{a \,. \, . \, b}\right]$ be a closed real interval.

Let $f: \left[{a \,. \, . \, b}\right] \to \R$ be a bounded real function.

Let $P = \left\{{x_0, x_1, x_2, \ldots, x_n}\right\}$ be a subdivision of $\left[{a \,. \, . \, b}\right]$.

For all $\nu \in \left\{{1, 2, \ldots, n}\right\}$, let $m_\nu^{\left({f}\right)}$ be the infimum of $f$ on the interval $\left[{x_{\nu - 1} \,. \, . \, x_{\nu}}\right]$.

Then:
 * $\displaystyle L^{\left({f}\right)} \left({P}\right) = \sum_{\nu \mathop = 1}^n m_\nu^{\left({f}\right)} \left({x_{\nu} - x_{\nu - 1}}\right)$

is called the '''lower sum of $f$ on $\left[{a \,. \, . \, b}\right]$ belonging (or with respect) to (the subdivision) $P$'''.

If there is no ambiguity as to what function is under discussion, $m_\nu$ and $L \left({P}\right)$ are often seen.

Also known as
The lower sum is also known as the lower Darboux sum or the lower Riemann sum.

Also see

 * Upper Sum
 * Upper Integral
 * Lower Integral
 * Riemann Sum