Morse index and nullity of rotationally symmetric $Y$-singular minimal surfaces

Abstract

We compute the Morse index and nullity of a one-parameter family of rotationally symmetric minimal $Y$-singular surfaces in ${\mathbb{R}}^3$ whose singular set is a circle. The second variation is studied on a weighted energy space adapted to the noncompact faces and decomposed into angular Fourier modes. The resulting modewise problem separates the fixed-junction Dirichlet contribution from a finite-dimensional junction form. The exceptional resonant radial mode is treated directly. We prove that the $Y$-catenoid has index one and nullity three, while $Y_{\alpha }$ has index two and nullity five for $0<\alpha \le \pi /3$.