Hoàng Xuân Sính’s thesis: Categorifying Group Theory

During what Vietnamese call the American War, AlexanderGrothendieck spent three weeks teaching mathematics in and near Hanoi.Hoàng Xuân Sính took notes on his lectures and later did her thesis workwith him by correspondence. In her thesis she developed the theory of‘Gr-categories’, which are monoidal categories in which all objects andmorphisms have inverses. Now often called ‘2-groups’, these structuresallow the study of symmetries that themselves have symmetries. After abrief account of how Hoàng Xuân Sính wrote her thesis, we explain someof its main results, and its context in the history of mathematics.