The quantitative Faber-Krahn inequality for the Robin Laplacian
  We prove a quantitative Faber-Krahn inequality for the first eigenvalue of the Laplace operator with Robin boundary conditions. The asymmetry term involves the square power of the Fraenkel asymmetry, multiplied by a constant depending on the Robin parameter, the dimension of the space and the measure of the set. 
D. Bucur, V. Ferone, C. Nitsch, C. Trombetti
math.AP