Functional Analysis 61 Functional analysis Functions of real variables 61 Real Functions Operator Theory 14 Operator theory 14 Differential Equations 13 Differential equations 13 Mathematical analysis 13 Measure and Integration 12 Measure theory 12 Analysis 11 Functions of a Complex Variable 10 Functions of complex variables 10 Fourier Analysis 8 Fourier analysis 8 Approximation theory 7 Approximations and Expansions 7 Mathematical optimization 6 Optimization 6 Probabilities 5 Probability Theory 5 Convex and Discrete Geometry 4 Convex geometry  4 Discrete geometry 4 Integral Equations 4 Integral equations 4 Numerical Analysis 4 Numerical analysis 4 Abstract Harmonic Analysis 3