In this task, you will be presented with a text and a pronoun. You should write an implausible answer to the question of what is the pronoun's reference. Even though there exist multiple wrong answers, we only need a single wrong answer. Position of the pronoun in the text is showed within two "_"s.

Bernard Osgood Koopman (1900 -- August 18, 1981) was a French-born American mathematician, known for his work in ergodic theory, the foundations of probability, statistical theory and operations research. After living in France and Italy, the Koopmans immigrated to the United States in 1915. Koopman was a student of George David Birkhoff and _his_ initial work concentrated on dynamical system s and mathematical physics. <sep>, Pronoun: his
George David Birkhoff