Works citing   [Potential Anal. 52 (2020), 601-614]

1. R. S. Laugesen:
   The Robin Laplacian-Spectral conjectures, rectangular theorems,
   J. Math. Phys. 60 (2019), 121507.
   
   
2. A. V. Vikulova:
   Parallel coordinates in three dimensions and sharp spectral isoperimetric inequalities,
   Ricerche di Matematica (2020) .