One number tells regulators whether a market is dangerously concentrated

Square each firm's market share and add them up: the total runs from near zero, for a market of many equal-sized rivals, up to 10,000 for a pure monopoly. Economists Albert Hirschman and Orris Herfindahl arrived at the same formula independently, in 1945 and 1950. US antitrust regulators still use it today to flag mergers that would concentrate a market too far.

— Orris C. Herfindahl, Concentration in the Steel Industry — PhD dissertation, Columbia University, 1950