In mathematics, a function space is a set of functions that share a common domain and range, together with a mathematical structure that allows for operations such as addition, multiplication, and composition. If $x$ is a signal with domain $D$ and range $C$: $ x: D \rightarrow C $ If the system $S$ accepts signals of the type $x$ at its input, we can say that its domain is a **function space** $X$ to which $x$ belongs. We will represent a **function space** wrapping in square brackets the domain and range of the signals it represents. For example, the function space of the discrete-time signals can be represented as: $ X = [\mathbb{Z} \rightarrow \mathbb{R}] $