Lars Hörmander is a Swedish analyst and Fields medalist. His work in harmonic analysis, especially microlocal analysis and the analysis of linear PDEs was a very central contribution to this theory in the 1960s and 1970s. He wrote also several books including his magnus treatise
The analysis of linear partial differential operators, in 4 vols.: I. Distribution theory and Fourier analysis, II. Differential operators with constant coefficients, III. Pseudo-differential operators, IV. Fourier integral operators.