On the Index of the Critical Möbius Band in 𝔹4
In this paper, we prove that the Morse index of the critical Möbius band in the 4-dimensional Euclidean ball 𝔹4 equals 5. It is conjectured that this is the only embedded non-orientable free boundary minimal surface of index 5 in 𝔹4. One of the ingredients in the proof is a comparison theorem between the spectral index of the Steklov problem and the energy index. The latter also enables us to give another proof of the well-known result that the index of the critical catenoid in 𝔹3 equals 4.