Variable n' : nat.
Local Notation n := n'.+1.
Canonical matrix_algType :=
Eval hnf in AlgType R 'M[R]_n (fun k => scalemxAr k).
End MatrixAlgType.

Lemma cofactorZ n a (A : 'M[R]_n) i j :
cofactor (a *: A) i j = a ^+ n.-1 * cofactor A i j.