I am simulating the following discrete time feedback system:

How can I prove that it is stable for

K_\mathrm{u} < 2 \frac{1}{T_\mathrm{s} K_2}

?

Code:

```
using ControlSystemsBase
Ts = 0.01
Ku = 100/5e6
K2 = 5e6
num = [Ku * Ts]
den = [1, -1]
P = tf(num, den, Ts) # plant, the integrator
C = K2 # controller, a constant gain
sys = feedback(P, C)
```

Output of the command `margin`

:

```
julia> margin(sys)
(wgm = [NaN;;], gm = [Inf;;], wpm = [NaN;;], pm = [Inf;;])
```

Does not look like a good answerâ€¦