Free boundary minimal surfaces in products of balls

In this paper we develop an extremal eigenvalue approach to the problem of construction of free boundary minimal surfaces in the product of Euclidean balls of chosen radii.

The extremal problem involves a linear combination of normalized mixed Steklov-Neumann eigenvalues.

The problem is motivated by the Schwarz P-surface which is a free boundary minimal surface in a cube.

We show that the problem often does not have an absolute maximum in the product case even though it is bounded from above.

By imposing a finite group of symmetries on both the surface and on the eigenfunctions we construct at least one free boundary minimal surface in a rectangular prism with arbitrary side lengths.

We further show that unless the rectangular prism is a cube there are at least two such surfaces.

We also prove that any immersed free boundary minimal surface of genus 0 with one boundary component on each face of a rectangular prism, and that is invariant under the reflections interchanging opposite faces of the prism, is necessarily embedded.

Finally we show that for a genus 0 surface with 6 boundary components and suitable reflection symmetries there is a maximizing metric which can be realized by a free boundary minimal immersion into a product of Euclidean balls.