When the city was a world centre of mathematical thought.
Like many of you I’ve hardly been able to think about anything else these past ten days apart from the war in Ukraine. So today’s puzzles are a celebration of Lviv, Ukraine’s western city, which played an important role in the history of 20th century mathematics.
During the 1930s, a remarkable group of scholars came up with new ideas, methods and theorems that helped shape the subject for decades.