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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...
