Exterior free-boundary minimal surfaces of every topological type with at least two ends

Abstract

An exterior free-boundary minimal surface is a properly immersed minimal surface in the complement of the open unit ball of $\mathbb{R}^3$ whose boundary lies on the unit sphere and meets it orthogonally. We prove that for all integers $\ell\geq2$, $b\geq1$ and $γ\geq0$ there is a connected, orientable, properly embedded exterior free-boundary minimal surface with $\ell$ ends, $b$ boundary components and genus $γ$. Its total curvature is finite, and its ends are parallel catenoidal ends of multiplicity one. By the theorem of Mazet and Mendes, a surface of this kind with one end is a rotational annulus. Hence the topological types of such embedded surfaces are exactly the annulus with one end and all types with at least two ends. There are two gluing constructions. For $\ell\geq3$ we stack $\ell$ sheets over a stable catenoidal annulus that meets the sphere along a circle of radius $4/5$, and we join adjacent sheets by half-catenoidal bridges sitting on the sphere. The sheets are small normal graphs over this annulus whose displacements contain different multiples of the Jacobi field of the family of such annuli, so their boundary circles and the logarithmic growth of their ends differ slightly. For $\ell=2$ we double a catenoidal annulus near the critical angle $π/4$, where it acquires a bounded rotationally invariant Jacobi field; the distance to the critical annulus is coupled to the size of the bridges. In gluing constructions inside the unit ball that double or stack a compact surface, the bridges themselves must balance the sheets, which forces many bridges. Here the logarithmic growth of the ends absorbs this balancing. A fixed number of bridges therefore suffices, and every topological type is reached, including the small ones.