A first-order formulation for axisymmetric Willmore surfaces

Abstract

We show that axisymmetric Willmore surfaces admit a first-order formulation obtained by combining two independent first integrals. If $ρ$ denotes the distance from the axis of revolution and $Ψ=\sinψ$, where $ψ$ is the tangent angle of the generating curve, then the profile satisfies \begin{equation*} \left[\frac{Ψ(ρΨ'-Ψ)^2+2(ρΨ'-Ψ)+2C_1ρ}{\sqrt{1-Ψ^2}}\right]^2 +\left[(ρΨ'-Ψ)^2-2\right]^2=C_2, \end{equation*} where $C_1$ and $C_2$ are constants of integration and the prime denotes differentiation with respect to $ρ$. This equation reduces the axisymmetric Willmore equation to a first-order ordinary differential equation and provides a convenient classification scheme for Willmore surfaces of revolution. The sphere and the Clifford torus are discussed as elementary checks of the formulation.