Back in the 1940s, George Zipf, an American researcher, noted that a city’s population is inversely proportional to its rank in a country. His law holds that the largest city is always about twice as big as the second largest, three times as big as the third largest, and so on. Other regularities have emerged since. Big cities decentralise as they grow, creating more jobs outside the centre. Urban population density in all industrialised countries declines slowly as you move away from the centre. (Moscow, exceptionally, is the other way round.)
