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)=xn+an−1xn−1+...+a−1x+a0,
this matrix is equal to the unit matrix of size n−1 bordered with
[0,0..,0,−a0] as first row, and with
[−a0,−a1,....,−an−1] as last column.
Input :
^
2+5x-7,x)Output :
Input :
^
4+3x^
3+2x^
2+4x-1,x)Output :