We prove that extremals for second order Rellich-Sobolev inequalities have a constant sign. Then, we show that the optimal constants in Rellich-Sobolev inequalities on a bounded domain $\Omega$ and under Navier boundary conditions do not depend on $\Omega$.
Roberta Musina. "Optimal Rellich-Sobolev constants and their extremals." Differential Integral Equations 27 (5/6) 579 - 600, May/June 2014. https://doi.org/10.57262/die/1396558098