Complex sentence: With Laurent Schwartz he supervised the early research of Alexander Grothendieck; later from 1959 to 1964 he was at IHÉS alongside Grothendieck, and collaborating on the expository work needed to support the project of refounding algebraic geometry on the new basis of schemes.
Split into simpler sentences: With Laurent Schwartz he supervised the early research of Alexander Grothendieck. Later from 1959 to 1964 he was at the Institut des Hautes Études Scientifiques alongside Grothendieck, and collaborating on the expository work needed to support the project of refounding algebraic geometry on the new basis of schemes.