Herman ring
In the mathematical discipline known as complex dynamics, the Herman ring is a Fatou component where the rational function is conformally conjugate to an irrational rotation of the standard annulus. == Formal definition == Namely if ƒ possesses a Herman ring U with period p, then there exists a conformal mapping ϕ : U → { ζ : 0 < r < | ζ | < 1 } {\displaystyle \phi :U\rightarrow \{\zeta :0<r<|\zeta |<1\}} and an irrational number θ {\displaystyle \theta } , such that ϕ ∘ f ∘ p ∘ ϕ − 1 ( ζ ) = e 2 π i θ ζ .