mathematician who wrote a pioneering memoir on topology, the study of surfaces and higher-dimensional spaces, and wrote one of the first rigorous expositions of the theory of equations developed by the noted French mathematician Évariste Galois (Galois, Évariste) (1811–32).

Betti studied mathematics and physics at the University of Pisa. After graduating with a degree in mathematics in 1846, he stayed on to work as an assistant until 1849 when he returned home to Pistoia to teach at a secondary school. From 1854 he taught at another secondary school in Florence. In 1857 he obtained a professorship in mathematics at Pisa, where he remained for the rest of his academic life. He fought in two battles for Italian independence and was elected to the new Italian parliament in 1862.

Betti's early work was in the theory of equations and algebra. He extended and furnished proofs for Galois's work, which had previously been stated in part without demonstrations or proofs. (Before Galois could finish his work, he died in a duel at age 21.) The arrival in Pisa of the German mathematician Bernhard Riemann (Riemann, Bernhard) in 1863 decisively affected the course of Betti's research. They became close friends, and Riemann awakened Betti's interest in mathematical physics, in particular potential theory and elasticity, and inspired his memoir on topology. Betti's study of spaces of higher dimensions (greater than three) in the latter work did much to open up the subject, and it led the French mathematician Henri Poincaré (Poincaré, Henri) to give the name Betti numbers to certain numbers that characterize the connectivity of a manifold (the higher-dimensional analog of a surface).