Let $f$ be the D1672: Collatz function.

Then \begin{equation} 1 \in \bigcap_{n \in 1, 2, 3, \ldots} \{ n, f(n), f(f(n)), \dots \} \end{equation}

Let $f$ be the D1672: Collatz function.

Then \begin{equation} \forall \, n_0 \in 1, 2, 3, \ldots : \exists \, n \in \mathbb{N} : f^n(n_0) = 1 \end{equation}