Analyse harmonique Asymptotic distribution of eigenvalues and eigenfunctions Difference and functional equations Differential equations, Partial Existence, uniqueness, and regularity theory [See also 35Q30] Équations aux dérivées partielles Fluid mechanics {For general continuum mechanics, see 74Axx, or other parts of 74-XX} Function spaces arising in harmonic analysis Functional analysis {For manifolds modeled on topological linear spaces, see 57Nxx, 58Bxx} Functional equations and inequalities [See also 30D05] General General higher-order equations and systems Geometric optics Harmonic analysis Harmonic analysis in several variables {For automorphic theory, see mainly 11F30} Harmonic analysis on Euclidean spaces Incompressible viscous fluids Integral representations, integral operators, integral equations methods Linear function spaces and their duals [See also 30H05, 32A38, 46F05] {For function algebras, see 46J10} Nonlinear higher-order equations Optics, electromagnetic theory {For quantum optics, see 81V80} Partial differential equations Potential theory {For probabilistic potential theory, see 60J45} Pseudodifferential operators Pseudodifferential operators and other generalizations of partial differential operators [See also 47G30, 58J40] Special functions (33-XX deals with the properties of functions as functions) {For orthogonal functions, see 42Cxx; for aspects of combinatorics see 05Axx; for number-theoretic aspects see 11-XX; for Spectral theory and eigenvalue problems [See also 47Axx, 47Bxx, 47F05] Systems of functional equations and inequalities Two-dimensional theory