On simplicity of spectra of p-biharmonic equations Original Research Article

In this paper, we deal with the nonlinear fourth-order boundary value problem with general Robin-type boundary conditions View the MathML source where View the MathML source, p,q>1, M,N are 2×4 matrices, View the MathML source, and View the MathML source Under some restrictions on M and N, we show that every positive eigenvalue λ is simple. On the other hand, we give a counterexample showing that without these restrictions it need not be true. We also discuss the question of positivity of all eigenvalues