Definition:Real Interval/Notation

Definition
An arbitrary (real) interval is frequently denoted $\mathbb I$.

Sources which use the $\textbf {boldface}$ font for the number sets $\N, \Z, \Q, \R, \C$ tend also to use $\mathbf I$ for this entity.

Some sources merely use the ordinary $\textit {italic}$ font $I$.

Some sources prefer to use $J$.

Reverse-Bracket Notation
Some sources use a deliberately explicit notation, along the lines:


 * $I: a < x < b$ to denote $\openint a b$


 * $I: a \le x \le b$ to denote $\closedint a b$


 * $I: a \le x < b$ to denote $\hointr a b$

For the sake of convenience, the $I$ is often omitted, for example:
 * $a < x < b$ to denote $\openint a b$

However, this notation is discouraged on.