Abraham Plessner

Abraham Ezechiel Plessner (1900 to 1961) was a mathematician born in Lodz, then in the Russian Empire and now in Poland, who is regarded as a founder of the Moscow school of functional analysis. He studied at Giessen under Ludwig Schlesinger and Friedrich Engel, attended courses at Gottingen and Berlin, and took his doctorate at Giessen in 1922 with a thesis on conjugate trigonometric series. While working in Marburg he edited Leopold Kronecker collected works with Kurt Hensel and published two results that now bear his name as Plessner theorems, one on the convergence of conjugate trigonometric series and one on the boundary behaviour of functions meromorphic in the unit disc, which also gave rise to the idea of a Plessner point. With Schlesinger he wrote a textbook on Lebesgue integrals and Fourier series that appeared in 1926. The University of Giessen refused him the right to lecture because he was a Russian citizen, so in 1930 he moved to Moscow and joined the group of Nikolai Luzin. There his interests turned to spectral theory of operators in Hilbert space. He became a professor in 1939, was dismissed from both his posts in 1949, and died in 1961, leaving a book on the spectral theory of linear operators that colleagues finished after his death.