HHI = ∑ Si. Si as it equals the sum of the squared market shares of each firm in the industry. (See Friday, March 10, 2006) = -ε. (∑ Si. Si )/(-ε) (multiply and divide with -ε) = -ε.∑Si.L, where L is the Lerner Index of market power in an oligopoly environment (See Friday, March 17, 2006). That is, the HHI equals the absolute value of the market demand elasticity multiply with the weighted average of the firms’ price-cost margins.