A function with constant elasticity of substitution. CES is popular for both production and utility functions. Used extensively in New Trade Theory as the Dixit-Stiglitz utility function for differentiated products under monopolistic competition. With arguments X = (X₁,. . . ,X_n), the function is F(X) = A(_ia_iX_i)^1/, where a_i, A are positive constants and  = 1/(1-) is the elasticity of substitution. Due to Arrow et al. (1961).