Steklov eigenvalues and quasiconformal maps of simply connected planar   domains
  We investigate isoperimetric upper bounds for sums of consecutive Steklov eigenvalues of planar domains. The normalization involves the perimeter and scale-invariant geometric factors which measure deviation of the domain from roundness. We prove sharp upper bounds for both starlike and simply connected domains, for a large collection of spectral functionals including partial sums of the zeta function and heat trace. The proofs rely on a special class of quasiconformal mappings. 
A. Girouard, R. S. Laugesen and B. A. Siudeja
math.SP math.AP