# scipy.signal.invresz¶

scipy.signal.invresz(r, p, k, tol=0.001, rtype='avg')[source]

Compute b(z) and a(z) from partial fraction expansion.

If M is the degree of numerator b and N the degree of denominator a:

        b(z)     b[0] + b[1] z**(-1) + ... + b[M] z**(-M)
H(z) = ------ = ------------------------------------------
a(z)     a[0] + a[1] z**(-1) + ... + a[N] z**(-N)


then the partial-fraction expansion H(z) is defined as:

        r[0]                   r[-1]
= --------------- + ... + ---------------- + k[0] + k[1]z**(-1) ...
(1-p[0]z**(-1))         (1-p[-1]z**(-1))


If there are any repeated roots (closer than tol), then the partial fraction expansion has terms like:

     r[i]              r[i+1]                    r[i+n-1]
-------------- + ------------------ + ... + ------------------
(1-p[i]z**(-1))  (1-p[i]z**(-1))**2         (1-p[i]z**(-1))**n


This function is used for polynomials in negative powers of z, such as digital filters in DSP. For positive powers, use invres.

Parameters
rarray_like

Residues corresponding to the poles. For repeated poles, the residues must be ordered to correspond to ascending by power fractions.

parray_like

Poles. Equal poles must be adjacent.

karray_like

Coefficients of the direct polynomial term.

tolfloat, optional

The tolerance for two roots to be considered equal in terms of the distance between them. Default is 1e-3. See unique_roots for further details.

rtype{‘avg’, ‘min’, ‘max’}, optional

Method for computing a root to represent a group of identical roots. Default is ‘avg’. See unique_roots for further details.

Returns
bndarray

Numerator polynomial coefficients.

andarray

Denominator polynomial coefficients.

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scipy.signal.invres