Operator theory Functional analysis 9 Spectral theory (Mathematics) 6 Hilbert space 5 Banach algebras 3 Congresses 3 Linear operators 3 Mathematics 3 Matrices 3 Algebra 2 Approximation theory 2 Approximations and expansions 2 C*-algebras 2 Differential equations, Partial 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 Integral equations 2 MATHEMATICS / Algebra / General 2 MATHEMATICS / Functional Analysis 2 Mathematical analysis 2 Mathematical physics.1 2 Number theory 2 Operator Theory 2 Probability theory and stochastic processes 2 Real functions 2 System theory 2