# Definition:Open Rectangle

## Definition

Let $n \ge 1$ be a natural number.

Let $a_1, \ldots, a_n, b_1, \ldots, b_n$ be real numbers.

$\ds \prod_{i \mathop = 1}^n \openint {a_i} {b_i} = \openint {a_1} {b_1} \times \cdots \times \openint {a_n} {b_n} \subseteq \R^n$

is called an open rectangle in $\R^n$ or open $n$-rectangle.

The collection of all open $n$-rectangles is denoted $\JJ_o$, or $\JJ_o^n$ if the dimension $n$ is to be emphasized.

### Degenerate Case

In the case where $a_i > b_i$ for some $i$, the open rectangle $\ds \prod_{i \mathop = 1}^n \openint {a_i} {b_i}$ degenerates to the empty set $\O$.

This is in accordance with the result Cartesian Product is Empty iff Factor is Empty for general Cartesian products.

## Notation

A convenient notation for the open rectangle $\ds \prod_{i \mathop = 1}^n \openint {a_i} {b_i}$ is $\openrect {\mathbf a} {\mathbf b}$.

## Also see

• Results about open rectangles can be found here.

## Technical Note

The $\LaTeX$ code for $\openrect {\mathbf a} {\mathbf b}$ is \openrect {\mathbf a} {\mathbf b} .

This is a custom $\mathsf{Pr} \infty \mathsf{fWiki}$ command designed to implement Wirth interval notation and its derivatives.