Question: Which mathematician joined Yale in 1991, Grigory Margulis or Boris Galerkin?
Context: In mathematics, the multiplicative ergodic theorem, or Oseledets theorem provides the theoretical background for computation of Lyapunov exponents of a nonlinear dynamical system.  It was proved by Valery Oseledets (also spelled "Oseledec") in 1965 and reported at the International Mathematical Congress in Moscow in 1966.  A conceptually different proof of the multiplicative ergodic theorem was found by M. S. Raghunathan.  The theorem has been extended to semisimple Lie groups by V. A. Kaimanovich and further generalized in the works of David Ruelle, Grigory Margulis, Anders Karlsson, and François Ledrappier. Brian Tompkins is the former coach of Yale Varsity Soccer.  He joined Yale in 1996 and helped Yale to win its first Ivy League title since 1991, in 2005.  Before he joined Yale, Tompkins had also built the University of Wisconsin–Milwaukee into a national soccer power.  It was announced in August that 2014 would be his last year coaching, and that he would be moving into the administration following the season. Gregori Aleksandrovich Margulis (Russian: Григо́рий Алекса́ндрович Маргу́лис , first name often given as Gregory, Grigori or Grigory; born February 24, 1946) is a Russian-American mathematician known for his work on lattices in Lie groups, and the introduction of methods from ergodic theory into diophantine approximation.  He was awarded a Fields Medal in 1978 and a Wolf Prize in Mathematics in 2005, becoming the seventh mathematician to receive both prizes.  In 1991, he joined the faculty of Yale University, where he is currently the Erastus L. De Forest Professor of Mathematics. In differential geometry, a subfield of mathematics, the Margulis lemma (named after Grigory Margulis) is a result about discrete subgroups of isometries of a non-positively curved Riemannian manifolds (e.g. the hyperbolic n-space).  Roughly, it states that within a fixed radius, usually called the Margulis constant, the structure of the orbits of such a group cannot be too complicated.  More precisely, within this radius around a point all points in its orbit are in fact in the orbit of a nilpotent subgroup (in fact a bounded finite number of such). In mathematics, Ratner's theorems are a group of major theorems in ergodic theory concerning unipotent flows on homogeneous spaces proved by Marina Ratner around 1990.  The theorems grew out of Ratner's earlier work on horocycle flows.  The study of the dynamics of unipotent flows played a decisive role in the proof of the Oppenheim conjecture by Grigory Margulis.  Ratner's theorems have guided key advances in the understanding of the dynamics of unipotent flows.  Their later generalizations provide ways to both sharpen the results and extend the theory to the setting of arbitrary semisimple algebraic groups over a local field. Douglas Clyde Macintosh (1877–1948) was a theologian and academic.  Macintosh was born in Canada in 1877 and received his undergraduate degree from McMaster University when it was in Toronto.  In 1907 was ordained a Baptist minister and taught at Brandon College, Manitoba.  In 1909 Macintosh received his Ph.D from the University of Chicago and joined Yale Divinity School, becoming an assistant professor of systematic theology. Boris Grigoryevich Galerkin (Russian: Бори́с Григо́рьевич Галёркин , surname more accurately romanized as Galyorkin; 4 March [O.S. 20 February] 1871  – 12 July 1945), born in Polotsk, Vitebsk Governorate, Russian Empire, was a Jewish Soviet mathematician and an engineer. In mathematics, in the theory of discrete groups, superrigidity is a concept designed to show how a linear representation ρ of a discrete group Γ inside an algebraic group "G" can, under some circumstances, be as good as a representation of "G" itself.  That this phenomenon happens for certain broadly defined classes of lattices inside semisimple groups was the discovery of Grigory Margulis, who proved some fundamental results in this direction. Phyllis Emily Granoff (born 1947) is a specialist in Indic religions.  In July 2004 she joined Yale University as a Professor of World Religions.  She also serves as the editor of the Journal of Indian Philosophy. The napkin folding problem is a problem in geometry and the mathematics of paper folding that explores whether folding a square or a rectangular napkin can increase its perimeter.  The problem is known under several names, including the Margulis napkin problem, suggesting it is due to Grigory Margulis, and the Arnold's rouble problem referring to Vladimir Arnold and the folding of a Russian ruble bank note.  Some versions of the problem were solved by Robert J. Lang, Svetlana Krat, Alexey S. Tarasov, and Ivan Yaschenko.  One form of the problem remains open.
Answer: Gregori Aleksandrovich Margulis