Dedekind made notable contributions to set theory and number theory. Gauss’s last student, he was was also taught by Weber. Riemann was a fellow-student, lifelong friend and professional colleague; visiting Berlin together, they met Borchardt, Kummer and Weierstrass. Gauss’s successor Dirichlet also became a close friend as well as professional colleague; Dedekind published the work of both, and of Riemann, posthumously. Cantor was met on holiday in Switzerland; they became correspondents and friends for life, although it has been suggested that latterly they were unnecessarily reserved about sharing ideas with one another.