Oskar Benjamin Klein was a Swedish theoretical physicist.

Paul Ehrenfest was an Austrian theoretical physicist who made major contributions to the field of statistical mechanics and its relations with quantum mechanics, including the theory of phase transition and the Ehrenfest theorem. He befriended Albert Einstein on a visit to Prague in 1912 and became a professor in Leiden, where he frequently hosted Einstein. He died by murder-suicide in 1933; he killed his disabled son Wassik, and then himself.