### 2.46.7  Characteristic polynomial using Hessenberg algorithm :pcar_hessenberg

pcar_hessenberg takes as argument a square matrix A of size n and optionnaly the name of a symbolic variable.
pcar_hessenberg returns the characteristic polynomial P of A written as the list of its coefficients if no variable was provided or written in its symbolic form with respect to the variable name given as second argument, where

 P(x)=det(xI−A)

The characteristic polynomial is computed using the Hessenberg algorithm (see e.g. Cohen) which is more efficient (O(n3) deterministic) if the coefficients of A are in a finite field or use a finite representation like approximate numeric coefficients. Note however that this algorithm behaves badly if the coefficients are e.g. in ℚ.
Input :

pcar_hessenberg([[4,1,-2],[1,2,-1],[2,1,0]] % 37)

Output :

[1 % 37 ,-6% 37,12 % 37,-8 % 37]

Input :

pcar_hessenberg([[4,1,-2],[1,2,-1],[2,1,0]] % 37,x)

Output :

x`^`3-6 %37 *x`^`2+12 % 37 *x-8 % 37

Hence, the characteristic polynomial of [[4,1,-2],[1,2,-1],[2,1,0]] in ℤ/37 ℤ is

 x3−6x2+12x−8