[Python] Why is the numerator coefficient array returned by scipy.signal.cont2discrete two...

I'm converting a continuous-time dynamical system to discrete time using the function cont2discrete from Scipy's signal processing library.

dt = 0.1
num, den, dt = scipy.signal.cont2discrete(([1], [5, 1]), dt)
print(num, den)


Output:

[[0. 0.01980133]] [ 1. -0.98019867]


I then want to simulate this using lfilter, but lfilter takes a, b as arguments which are the numerator/denominator coefficient vectors "in a 1-D sequence."

So I need to do this:

u = np.ones(50)
y = scipy.signal.lfilter(num[0], den, u)


I'm just wondering why the convention is different for num and den, and for these two functions.

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