The Spectral Position of Neumann Domains on the Torus

Neumann domains of Laplacian eigenfunctions form a natural counterpart of nodal domains. The restriction of an eigenfunction to one of its nodal domains is the first Dirichlet eigenfunction of that domain. This simple observation is fundamental in many works on nodal domains. We consider a similar property for Neumann domains. Namely, given a Laplacian eigenfunction f and its Neumann domain $$\Omega $$, what is the position of $$\left. f\right| _{\Omega }$$ in the Neumann spectrum of $$\Omega $$? The current paper treats this spectral position problem on the two-dimensional torus. We fully solve it for separable eigenfunctions on the torus and complement our analytic solution with numerics for random waves on the torus. These results answer questions from (Band and Fajman in Ann Henri Poincare, 17(9):2379–2407, 2016; Zelditch in Surv Differ Geom 18:237–308, 2013) and raise new ones.