Definition:Open Rectangle

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

The set:


 * $\displaystyle \prod_{i=1}^n \left({a_i \,.\,.\, b_i}\right) = \left({a_1 \,.\,.\, b_1}\right) \times \cdots \times \left({a_n \,.\,.\, b_n}\right) \subseteq \R^n$

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

Here, $\times$ denotes Cartesian product.

In case $a_i \ge b_i$ for some $i$, the rectangle is taken to be the empty set $\varnothing$.

This is in accordance with the result Cartesian Product Null for general Cartesian products.

Alternative Notation
Some authors write $\left(({\mathbf a \,.\,.\, \mathbf b}\right))$ for $\displaystyle \prod_{i=1}^n \left({a_i \,.\,.\, b_i}\right)$ as a convenient abbreviation.