# Mikhail Yakovlevich Suslin

He contributed greatly to the theory of analytic sets, sometimes called after him, a kind of a set of reals which is definable via trees. In fact, while he was a research student of Nikolai Luzin (in 1917) he found an error in an argument of Lebesgue, who believed he had proved that for any Borel set in $\R^2$ (i. e. the analytic sets), the projection onto the real axis was also a Borel set.