Johann Bernoulli taught Euler unofficially; his friend and fellow-student Daniel Bernoulli helped persuade Euler’s father that his great gift was for mathematics, collaborated with him, and invited him to settle in St. Petersburg, where Goldbach was among his colleagues. Condorcet and Euler had an extensive working correspondence: other correspondents included Lomosonov, Clairaut, d’Alembert, Legendre, and the unreliable Stirling. Euler deputised for Maupertuis in Berlin. When Lagrange wrote to him with a new kind of calculus, he withheld his own work to let the 19-year-old get the credit. Lexell helped him when he was virtually blind.