pol2des polynomial matrix to descriptor form

## Syntax

    [N, B, C] = pol2des(Ds)

## Arguments

Ds

  polynomial matrix

N, B, C

  three real matrices

## Description

Given the polynomial matrix `Ds=D_0 +D_1 s +D_2 s^2 +... +D_k s^k`, `pol2des` returns three matrices `N, B, C`, with `N` nilpotent such that:

`Ds = C (s*N-eye())^-1 B`

## Examples

    s = poly(0,'s');
    G = [1, s; 1+s^2, 3*s^3];
    [N, B, C] = pol2des(G);
    G1 = clean(C*inv(s*N-eye())*B)
    G2 = G1.num

## See also

- ss2des
- tf2des