Schrödinger operator. Quantum theory--Mathematics. Quantum theory -- Instructional exposition (textbooks, tutorial papers, etc.). Quantum theory -- General mathematical topics and methods in quantum theory -- General mathematical topics and methods in quantum theory. Functional analysis -- Instructional exposition (textbooks, tutorial papers, etc.). Ordinary differential equations -- Boundary value problems -- Boundary value problems. Operator theory -- Special classes of linear operators -- Symmetric and selfadjoint operators (unbounded).