Herman ring
= Herman ring
{c}
A Herman ring is a doubly connected periodic <Fatou component> on which the first-return map is holomorphically conjugate to an irrational rotation of an annulus $r<|w|<1$. Compactness of the circle of rotation factors proves normality of the return iterates, and the finitely many intervening iterates preserve normality. It is the annular rotation-domain counterpart of a <Siegel disc>.