 of second-order sections

Parameters
----------
sos : array_like
    Array of second-order filter coefficients, must have shape
    ``(n_sections, 6)``. See `sosfilt` for the SOS filter format
    specification.

Returns
-------
b : ndarray
    Numerator polynomial coefficients.
a : ndarray
    Denominator polynomial coefficients.

Notes
-----
.. versionadded:: 0.16.0

Examples
--------
Find the polynomial representation of an elliptic filter
using its 'sos' (second-order sections) format.

>>> from scipy.signal import sos2tf
>>> from scipy import signal
>>> sos = signal.ellip(1, 0.001, 50, 0.1, output='sos')
>>> sos2tf(sos)
(   array([0.91256522, 0.91256522, 0.        ]),
    array([1.        , 0.82513043, 0.        ]))
