proposition: Lemma expand_det_col n (A : 'M[R]_n) j0 : \det A = \sum_i (A i j0 * cofactor A i j0).
proof: Proof. rewrite -det_tr (expand_det_row _ j0). by under eq_bigr do rewrite cofactor_tr mxE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup perm order) mathcomp(div prime binomial ssralg countalg finalg zmodp)