Suppose the 100 largest cities in the United States in 1920 are ranked according to magnitude (areas of cities). From Lotka,6 the following relation holds approximately PR0.93 = 5,000,000 Here, P is the population of the city having respective rank R. This relation is called the law of urban concentration for 1920. Solve for P in terms of R, and then find how fast the population is changing with respect to rank