# Correct notation for multiple intervals for a function's range

I'm hoping that this isn't an obvious question, but I was curious about possible notations for what interval a piecewise function is defined on. For example, consider the following function (where $x \in \mathbb{R}$):

$$\begin{cases} x & 0 \leq x \leq 5 \\ x^2 & 5 < x < 10 \\ x^3 & 10 < x \le 20 \\ 0 & x < 0 \lor x > 20 \end{cases}$$

The output is on the intervals $[0, 5]$ (when $0 \leq x \leq 5$), $(25, 100)$ (when $5 < x < 10$), and $(1000, 8000]$ when $10 < x \le 20$. Is there a "better" way of notating just the interval of the output? For example, would writing something like $[[0, 5], (25, 100), (1000, 8000]]$ have any particular meaning? For the record, yes, I do realize that this proposed notation actually has somewhat less information than the original sentence because I'm not specifying when each interval will actually "occur," but suppose that I don't care about specifying that for what I'm communicating.

The original context for this was a Stack Overflow post asking how to extend a set of numbers in the range of [0, 1] to a set in the range [-1, 1] (where $[0, 0.5) \to [-1, 0]$ and $[0.5, 1] \to [0, 1]$. One proposed answer was

$$\begin{cases} x - 1 & 0 \leq x < 0.5 \\ x & 0.5 \le x \le 1 \end{cases}$$

Clearly, this is actually on the range of $(-0.5, 0]$ (if ($0 \le x < 0.5$) or $[0.5, 1]$ (if $0.5 \le x \le 1$).

What is the most concise notation for stating what the actual interval of this proposed function is?

In your example $[[0, 5], (25, 100), (1000, 8000]]$, one way of writing this is $$[0,5]\cup(25,100)\cup(1000,8000].$$
$\cup$ means union (of sets).