Darboux integral

The Darboux integral is a formulation of integration in real analysis defined using upper and lower sums over partitions of an interval. It provides an order-theoretic approach to integration and is equivalent to the Riemann integral.

Definition

Darboux sums

Let $f:[a,b]\to ℝ$ be a bounded function. Let $$P=\{x_0,x_1,\dots ,x_n\},a=x_0<x_1<\cdots <x_n=b$$ be a partition of the interval $[a,b]$.

For each subinterval $[x_{i-1},x_i]$ of $P$, define:

• the infimum:

$$m_i=\underset{x\in [x_{i-1},x_i]}{\inf }f(x),$$

• the supremum:

$$M_i=\underset{x\in [x_{i-1},x_i]}{\sup }f(x).$$

The lower Darboux sum of $f$ with respect to $P$ is $$L(f,P)=\sum _{i=1}^n{m}_i(x_i-x_{i-1}),$$

and the upper Darboux sum is $$U(f,P)=\sum _{i=1}^n{M}_i(x_i-x_{i-1}).$$

The Darboux integral

Let $𝒫([a,b])$ denote the set of all partitions of $[a,b]$.

The lower Darboux integral of $f$ on $[a,b]$ is defined by $$\underset{\_}{\underset{a}{\overset{b}{\int }}}f=\underset{P\in 𝒫([a,b])}{\sup }L(f,P).$$

and the upper Darboux integral is defined by $$\overline{\underset{a}{\overset{b}{\int }}}f=\underset{P\in 𝒫([a,b])}{\inf }U(f,P).$$

If the upper and lower Darboux integrals are equal, then the Darboux integral of $f$ on $[a,b]$ is defined by their common value, that is, $$\underset{a}{\overset{b}{\int }}f=\underset{\_}{\underset{a}{\overset{b}{\int }}}f=\overline{\underset{a}{\overset{b}{\int }}}f.$$ In this case, function $f$ is said to be Darboux-integrable.

See also

• Riemann integral
• Partition

Terminology

en	fr	de	zh		ja
Darboux integral	intégrale de Darboux	Darboux-Integral	Darboux 积分	Darboux 積分	ダルブー積分
Darboux sum	somme de Darboux	Darboux-Summe	Darboux 和	Darboux 和	ダルブー和
upper Darboux sum	somme supérieure de Darboux	obere Darboux-Summe	上 Darboux 和	上 Darboux 和	上ダルブー和
lower Darboux sum	somme inférieure de Darboux	untere Darboux-Summe	下 Darboux 和	下 Darboux 和	下ダルブー和
upper integral	intégrale supérieure	oberes Integral	上 Darboux 积分	上 Darboux 積分	上ダルブー積分
lower integral	intégrale inférieure	unteres Integral	下 Darboux 积分	下 Darboux 積分	下ダルブー積分

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