Section Groups.
Variable p : nat.
Definition Zp := if p > 1 then [set: 'Z_p] else 1%g.
Definition units_Zp := [set: {unit 'Z_p}].
Canonical units_Zp_group := [group of units_Zp].

Lemma card_units_Zp : p > 0 -> #|units_Zp| = totient p.