Let $\mathbb{N}$ be the D225: Set of natural numbers.
The Catalan sequence is the D997: Natural number sequence \begin{equation} \mathbb{N} \to \mathbb{N}, \quad n \mapsto \frac{1}{1 + n} \binom{2 n}{n} \end{equation}

Let $\mathbb{N}$ be the D225: Set of natural numbers.
The Catalan sequence is the D997: Natural number sequence \begin{equation} \binom{0}{0}, \quad \frac{1}{2} \binom{2}{1}, \quad \frac{1}{3} \binom{4}{2}, \quad \frac{1}{4} \binom{6}{3}, \quad \ldots \end{equation}