End SubDsumsmx.
Section Eigenspace.
Variables (n : nat) (g : 'M_n).
Definition eigenspace a := kermx (g - a%:M).
Definition eigenvalue : pred F := fun a => eigenspace a != 0.

Lemma eigenvalueP a :
reflect (exists2 v : 'rV_n, v *m g = a *: v & v != 0) (eigenvalue a).