An hypothesis in ecology proposed by George K Zipf that states that other factors such as income level, unemployment being equal, the number of persons who migrate between two communities (in both directions) will be equal to the product of the population of the two communities divided by the shortest distance between the communities. The hypothesis is often expressed in terms of the formula P1.P2/D in which P1 is the population of one community, P2 the population of the other community and D the shortest distance between the two communities. It is sometimes referred to as the P1P2/D hypothesis and also the hypothesis on the intercity movement of persons and the minimum equation hypothesis.