11. Schwarz’s proof can be found in Richard Courant and Herbert Robbins, What Is Mathematics?, Oxford University Press, 1941, pp. 346-49. A proof using the calculus was given by J. F. de’ Toschi di Fagnano (1715—97) in the Ada Eruditorum, 1775, p. 297. There were less elegant geometrical proofs before Schwarz’s.