<P> A general CES (Constant Elasticity of Substitution) form is </P> <Dl> <Dd> U (x, y) = (α x ρ + (1 − α) y ρ) 1 / ρ (\ displaystyle U (x, y) = \ left (\ alpha x ^ (\ rho) + (1 - \ alpha) y ^ (\ rho) \ right) ^ (1 / \ rho)) </Dd> </Dl> <Dd> U (x, y) = (α x ρ + (1 − α) y ρ) 1 / ρ (\ displaystyle U (x, y) = \ left (\ alpha x ^ (\ rho) + (1 - \ alpha) y ^ (\ rho) \ right) ^ (1 / \ rho)) </Dd> <P> where α ∈ (0, 1) (\ displaystyle \ alpha \ in (0, 1)) and ρ ≤ 1 (\ displaystyle \ rho \ leq 1). (The Cobb - Douglas is a special case of the CES utility, with ρ → 0 (\ displaystyle \ rho \ rightarrow 0 \,).) The marginal utilities are given by </P>

What does the slope of the indifference curve represent