Hilbert space 50 Functional analysis 9 Mathematics 9 Operator theory 6 Differential equations 4 Differential equations, Partial 4 Invariant subspaces 4 Quantum theory 4 Signal processing 4 Banach spaces 3 Calculus 3 Functional Analysis 3 Lecture Notes in Mathematics 3 Linear operators 3 Pitman Research Notes in Mathematics 3 Algebra 2 Algebraic Geometry 2 Approximation theory 2 Banach algebras 2 C*-algebras 2 Convex functions 2 Convex programming 2 Differential operators 2 Dirichlet principle 2 Dirichlet-Raum 2 Fourier transform-- Hilbert transform-- Laplace transform-- Integral transform 2 Holomorphic functions 2 Integral equations 2 Inverse problems (Differential equations) 2 Mathematical physics 2