After Grothendieck returned to France, he continued to teach Hoàng as a correspondence student.[5] She earned her doctorate under Grothendieck's supervision from Paris Diderot University in 1975, with a handwritten thesis.[6][7] Her thesis research, on algebraic structures based on categorical groups but with a group law that holds only up to isomorphism, prefigured much of the modern theory of 2-groups.[8]