High Quality Content by WIKIPEDIA articles! Paul Ehrenfest was an Austrian physicist and mathematician, who obtained Dutch citizenship on March 24, 1922. He 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. On December 21, 1904 he married Russian mathematician Tatyana Alexeyevna Afanasyeva (1876 1964), who collaborated with him in his work. They had two daughters and two sons: Tatyana ('Tanja') (1905 1984), also became a mathematician; Galinka ('Galja') (1910 1979), became an author and illustrator of children's books; Paul Jr. ('Pavlik') (1915 1939), who also became a physicist; and Vassily ('Wassik') (1918 1933), who had Down's Syndrome