Guido Fubini

Guido Fubini (1879–1943) was an Italian mathematician, brilliant and versatile, remembered for one supremely useful theorem — and for a life story that turned dramatic in his final years.

The great contribution

Fubini's theorem is the permission slip everyone uses when working with double integrals: under sensible conditions, you can integrate over a region one direction at a time, in whichever order is easier, and get the same answer. It sounds obvious, but proving exactly when you're allowed to swap the order is the whole point — and it's Fubini's name on the guarantee.

Fubini spent decades as one of Italy's leading mathematicians. Then, in the late 1930s, the Fascist regime's antisemitic laws stripped Jewish professors of their posts. Rather than live under them, Fubini — by then an old man in poor health — left everything behind and emigrated to the United States, where he taught at Princeton. He died a few years later, far from home, but free. It's a sober reminder that even the most abstract mathematicians live inside history, and sometimes history comes crashing in.