1 | initial version |

Your problem is that

```
# assume(omega, "real") # this does not work
```

thus prohibiting Sage to simplify expressions involving `real_part`

, `imag_part`

and others...

And this is attributable to

s = polygen(CC, "s")

omega = polygen(CC, "omega")

`s`

and `o`

no longer are symbolic expressions ; therefore, Sage can't `assume`

anything about them.

Tell me what's wrong with the trivial solution :

```
var("s")
s21 = 0.894292331092066/(s^4 + 2.14081977623463*s^3 + 3.15237117897600*s^2 + 2.31898630138664*s + 0.902488008823108)
var("omega", domain="real")
o21=s21.subs(s=I*omega)
(o21.diff()/o21).imag_part().plot(omega,-3,3)
```

whose result you can meditate upon...

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