Sergei Novikov proves in 1965 that the rational Pontryagin classes — invariants defined from a manifold's differentiable structure — are in fact topological invariants, that is, they depend only on the underlying topology and not on the chosen differentiable structure. The result, surprising for its time, is explicitly cited in his 1970 Fields Medal citation as his most outstanding contribution. From it, Novikov also formulates the conjecture bearing his name, on the homotopy invariance of certain higher characteristics derived from a manifold's fundamental group, a problem that remains an object of active research in topology decades later. Novikov was the first Soviet mathematician to receive the Fields Medal, although the USSR authorities prevented him from traveling to Nice to collect it in person.