Section MapFieldMatrix.
Variables (aF : fieldType) (rF : comUnitRingType) (f : {rmorphism aF -> rF}).
Local Notation "A ^f" := (map_mx f A) : ring_scope.

Lemma cormen_lup_lower n A (i j : 'I_n.+1) :
i <= j -> (cormen_lup A).1.2 i j = (i == j)%:R.