Operator theory 61 Functional analysis 14 Linear operators 13 Hilbert space 7 Spectral theory (Mathematics) 6 Congresses 5 Differential equations, Partial 5 Functional analysis-- Mathematics 4 Matrices 4 Banach algebras 3 Banach spaces 3 Hankel operators 3 Integral equations 3 Interpolation 3 Invariant subspaces 3 Mathematical analysis 3 Mathematics 3 Toeplitz operators 3 Topological and related algebra and groups 3 Algebra 2 Approximation theory 2 Approximations and expansions 2 C*-algebras 2 Function spaces 2 Functions of a complex variable 2 Functions of complex variables 2 Groupoids 2 Hardy spaces 2 Harmonic analysis on Euclidean spaces 2 Hilbert algebras 2