Theo Bauer · CC BY-SA 4.0

Proved foundational results in functional analysis and topology — the Hahn–Banach theorem, extending linear functionals in a way still taught in every graduate analysis course, bears his name alongside Stefan Banach's. As one of the founding figures of the Vienna Circle, he also helped define logical positivism, the movement that tried to ground philosophy in the same rigor he brought to mathematics. Died in 1934, before the movement's ideas spread further abroad ahead of the war.