English
If ZMod(n) is finite, its cardinal equals n.
Русский
Если ZMod(n) конечно, то кардинал равен n.
LaTeX
$$$\forall n,\ [Fintype(ZMod(n))] \Rightarrow Fintype.card(ZMod(n)) = n.$$$
Lean4
@[simp]
theorem card (n : ℕ) [Fintype (ZMod n)] : Fintype.card (ZMod n) = n := by
cases n with
| zero => exact (not_finite (ZMod 0)).elim
| succ n => convert Fintype.card_fin (n + 1) using 2