Armand Borel born in Switzerland. He worked on Lie groups, algebraic groups, and arithmetic groups, helping transform many areas of mathematics, including algebraic topology, differential geometry, algebraic geometry, and number theory.
Set theorist Ernst Zermelo died in Freiburg, Germany. He worked on statistical mechanics and the calculus of variations until becoming intrigued by Cantor's Continuum Hypothesis, posed as Hilbert's First Problem in 1900, and switching to set theory.