companion takes as argument an unitary polynomial P and the
name of its variable.

companion returns the matrix whose characteristic polynomial is P.

If P(x)=x^{n}+a_{n−1}x^{n−1}+...+a_{−1}x+a_{0},
this matrix is equal to the unit matrix of size n−1 bordered with
[0,0..,0,−a_{0}] as first row, and with
[−a_{0},−a_{1},....,−a_{n−1}] as last column.

Input :

companion(x

`^`

2+5x-7,x)Output :

[[0,7],[1,-5]]

Input :

companion(x

`^`

4+3x`^`

3+2x`^`

2+4x-1,x)Output :

[[0,0,0,1],[1,0,0,-4],[0,1,0,-2],[0,0,1,-3]]