Joseph Louis Lagrange

At 19, Lagrange wrote to Euler, proposing a new form of calculus. Poisson and he spurred each other on to refine planetary mathematics. D’Alembert supported him, Laplace was a mathematical correspondent and rival, and Lavoisier intervened to ensure that Lagrange, Italian-born, would not meet hostility in post-revolutionary Paris (Lavoisier was himself beheaded a few months later). Lagrange taught Fourier and encouraged Cauchy in his studies. A paper Germain sent led her to become (as a woman and outsider) his protégée. Lambert was a close friend, and Monge visited him when he was dying.