Career
He introduced the notion of proximity spaces at the First International Topological Conference in Moscow in 1934. He was imprisoned from 1937 to 1944, and did not publish on proximity spaces until 1951, at which point the theory was developed rapidly by Efremovič and associates. Yefremovich also introduced the notion of "volume invariants" for "equimorphisms" (that is, uniformly bicontinuous) on metric spaces.
These have proven to be very important in the study of manifolds and hyperbolic geometry.
