Invariant Seifert surfaces for strongly invertible knots

We study invariant Seifert surfaces for strongly invertible knots, and prove that the gap between the equivariant genus (the minimum of the genera of invariant Seifert surfaces) of a strongly invertible knot and the (usual) genus of the underlying knot can be arbitrary large. This forms a sharp contrast with Edmonds’ theorem that every periodic knot admits an invariant minimal genus Seifert surface. We also prove variants of Edmonds’ theorem, which are useful in studying invariant Seifert surfaces for strongly invertible knots.