Definition companionmx {R : ringType} (p : seq R) (d := (size p).-1) :=
\matrix_(i < d, j < d)
if (i == d.-1 :> nat) then - p`_j else (i.+1 == j :> nat)%:R.

Lemma mulmx_delta_companion (R : ringType) (p : seq R)
(i: 'I_(size p).-1) (i_small : i.+1 < (size p).-1):
delta_mx 0 i *m companionmx p = delta_mx 0 (Ordinal i_small) :> 'rV__.