arXiv, sa.No, ss.1-3, 2020 (Hakemsiz Dergi)
This note is written for a book dedicated to outstanding St-Petersburg mathematicians and timed to the ICM-2022 in St-Petersburg. In accordance with the plan of ICM-organizers, we try to tell about one of the most prominent Rokhlin's achievements in an accessible form and respecting the allowed volume.