On the Existence, Uniqueness, and Basis Properties of Radial Eigenfunctions of a Semilinear Second-Order Elliptic Equation in a Ball

We consider the following eigenvalue problem: , , , , where is an arbitrary fixed parameter and is an odd smooth function. First, we prove that for each integer there exists a radially symmetric eigenfunction which possesses precisely zeros being regarded as a function of . For sufficiently small, such an eigenfunction is unique for each . Then, we prove that if is sufficiently small, then an arbitrary sequence of radial eigenfunctions , where for each the th eigenfunction possesses precisely zeros in , is a basis in ( is the subspace of that consists of radial functions from . In addition, in the latter case, the sequence is a Bari basis in the same space.

Dissipative eigenvalue problems for a Sturm–Liouville operator with a singular potential

In this paper we consider the Sturm–Liouville operator d2/dx2 − 1/x on the interval [a, b], a < 0 < b, with Dirichlet boundary conditions at a and b, for which x = 0 is a singular point. In the two components L2(a, 0) and L2(0, b) of the space L2(a, b) = L2(a, 0) ⊕ L2(0, b) we define minimal symmetric operators and describe all the maximal dissipative and self-adjoint extensions of their orthogonal sum in L2(a, b) by interface conditions at x = 0. We prove that the maximal dissipative extensions whose domain contains only continuous functions f are characterized by the interface condition limx→0+(f′(x)−f′(−x)) = γf(0) with γ∈C+∪R or by the Dirichlet condition f(0+) = f(0−) = 0. We also show that the corresponding operators can be obtained by norm resolvent approximation from operators where the potential 1/x is replaced by a continuous function, and that their eigen and associated functions can be chosen to form a Bari basis in L2(a, b).

Spectral properties of the Orr-Sommerfeld problem

In this paper, we study the Orr–Sommerfeld problem on a finite interval. It is shown that the eigenfunctions and associated functions form a Bari basis in a suitable Hilbert space if the unperturbed velocity profile u is sufficiently smooth. To this end, the Orr–Sommerfeld problem is considered as a bounded perturbation of a certain self-adjoint spectral problem.