Let \(X\) be an abstract set. Then a map \(\mathbb{N}\to X\) is called sequence. The values are denoted by \(x_n\) for \(n\in \mathbb{N}\) and the whole series as:
\begin{equation*} (x_n)_{n\in \mathbb{N}} \qquad \text{or} \qquad (x_n). \end{equation*}If there is no ambiguity some authors even omit the parenthesis.