Section ModularGroup.
Variables p n : nat.
Let m := (p ^ n)%N.
Let q := (p ^ n.-1)%N.
Let r := (p ^ n.-2)%N.
Hypotheses (p_pr : prime p) (n_gt2 : n > 2).
Let p_gt1 := prime_gt1 p_pr.
Let p_gt0 := ltnW p_gt1.
Let def_n := esym (subnKC n_gt2).
Let def_p : pdiv m = p. Proof. by rewrite /m def_n pdiv_pfactor. Qed.
Let def_q : m %/ p = q. Proof. by rewrite /m /q def_n expnS mulKn. Qed.
Let def_r : q %/ p = r. Proof. by rewrite /r /q def_n expnS mulKn. Qed.
Let ltqm : q < m. Proof. by rewrite ltn_exp2l // def_n. Qed.
Let ltrq : r < q. Proof. by rewrite ltn_exp2l // def_n. Qed.
Let r_gt0 : 0 < r. Proof. by rewrite expn_gt0 ?p_gt0. Qed.
Let q_gt1 : q > 1. Proof. exact: leq_ltn_trans r_gt0 ltrq. Qed.

Lemma card_modular_group : #|'Mod_(p ^ n)| = (p ^ n)%N.