Luitzen Egbertus Jan Brouwer (1881-1966), usually cited as L. E. J. Brouwer, was a Dutch mathematician and philosopher, a graduate of the University of Amsterdam, who worked in topology, set theory, measure theory and complex analysis.
He is famous in topology for having proved the Brouwer fixed point theorem, here. He is the creator of intuitionism, a contructivist school of mathematics opposing the mathematics of Georg Cantor and the formalism of David Hilbert, here.