A classic work in market analysis was published by William J. Reilly under the title The Law of Retail Gravitation (New York: Knickerbocker Press, 1931).
Inspired by the formula for gravity, Reilly proposed that a similar formula could be used to calculate the point at which customers will be drawn to one or another of two competing centers.
The formula yields the break point between customers who will go to one center and those who will go to the other, located on a line connecting the two centers. The break point lies at distance xj from the smaller of the two centers. In the formula, dij is the total distance between the two centers, Pi is the size of the larger center, and Pj is the size of the smaller center.