Joseph-Louis Lagrange (1736–1813)
He was an Italian mathematician, physicist and astronomer, later naturalized French. He made significant contributions to the fields of analysis, number theory, and both classical and celestial mechanics.
Lagrange was not Germain's teacher, nor did he work with her directly. It is true that she studied Lagrange’s lecture notes and learned mathematics from his memoirs and other books at her disposal. But she did not have the benefit of Lagrange’s direct instruction.