We investigate the effect of the coefficient of the critical nonlinearity for the Neumann problem on the existence of least energy solutions. As a by-product we establish a Sobolev inequality with interior norm.
Chabrowski, J., & Willem, M. (2002). Least energy solutions of a critical Neumann problem with a weight. Calculus of Variations and Partial Differential Equations, 15(4), 421-431. https://doi.org/10.1007/s00526-002-0101-0 (Original work published 2002)