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Method

As can be seen from the code listing, this implementation of residuez simply calls residue, which was written to carry out the partial fraction expansions of $ s$-plane (continuous-time) transfer functions $ H(s)$:

\begin{eqnarray*} H(s) &=& \frac{B(s)}{A(s)}\\ &=& \frac{b_0 s^M + b_1 s^{M-1}... ...s^N + a_1 s^{N-1} + \cdots + a_{N-1}s + a_N}\\ &=& F(s) + R(s) \end{eqnarray*}
where $ F(s)$ is the ``quotient'' and $ R(s)$ is the ``remainder'' in the PFE:
$\displaystyle F(s)$ $\displaystyle \isdef$ $\displaystyle f_0 s^L + f_1 s^{L-1} + \cdots + f_{L-1} s + f_L$  
$\displaystyle R(s)$ $\displaystyle \isdef$ $\displaystyle \frac{r_1}{(s-p_1)^m_1} + \cdots + \frac{r_N}{(s-p_N)^m_N} \protect$ (J.1)

where $ L=M-N$ is the order of the quotient polynomial in $ s$, and $ m_i$ is the multiplicity of the $ i$th pole. (When all poles are distinct, we have $ m_i=1$ for all $ i$.) For $ M<N$, we define $ F(s)=0$. In the discrete-time case, we have the $ z$-plane transfer function
$\displaystyle H(z)$ $\displaystyle =$ $\displaystyle \frac{B(z)}{A(z)}$  
  $\displaystyle =$ $\displaystyle \frac{b_0 + b_1 z^{-1}+ \cdots + b_{M-1}z^{-(M-1)} + b_M z^{-M}} {a_0 + a_1 z^{-1}+ \cdots + a_{N-1}z^{-(N-1)} + a_N z^{-N}}. \protect$ (J.2)

For compatibility with Matlab's residuez, we need a PFE of the form $ H(z)=F(z)+R(z)$ such that
\begin{eqnarray*} F(z) &\isdef & f_0 + f_1 z^{-1}+ \cdots + f_{L-1} z^{-(L-1)} +... ...c{r_1}{(1-p_1z^{-1})^m_1} + \cdots \frac{r_N}{(1-p_Nz^{-1})^m_N} \end{eqnarray*}
where $ L=M-N$. We see that the $ s$-plane case formally does what we desire if we treat $ z$-plane polynomials as polynomials in $ z^{-1}$ instead of $ z$. From Eq.$ \,$(J.2), we see that this requires reversing the coefficient-order of B and A in the call to residue. In the returned result, we obtain terms such as

$\displaystyle \frac{\rho_i}{(s-\pi_i)^{m_i}} \isdef \frac{\rho_i}{(z^{-1}-\pi_i)^{m_i}} =\frac{(-1)^{m_i}\rho_i \pi_i^{-m_i}}{(1-\pi_i^{-1}z^{-1})^{m_i}} $

where the second form is simply the desired canonical form for $ z$-plane PFE terms. Thus, the $ i$th pole is

$\displaystyle p_i = \pi_i^{-1} $

and the $ i$th residue is

$\displaystyle r_i = \frac{\rho_i}{(-\pi_i)^{m_i}}. $

Finally, the returned quotient polynomial must be flipped for the same reason that the input polynomials needed to be flipped (to convert from left-to-right descending powers of $ z^{-1}$ [$ s$] in the returned result to ascending powers of $ z^{-1}$).
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Example with Repeated Poles
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The Padé-Prony Method