Definition 2.1.10 (Church numeral). Let $n$ be a natural number. Its corresponding Church numeral $c_{n}$ is the $λ$ -term $c_{n} : = λ s . λ z . s^{n} (z)$ , where $s^{n} (z)$ denotes

\underset{n times}{\underset{⏟}{s (s (\dots (s}} z) \dots) .