Below is the graph of a quadratic function, showing where the function is increasing and decreasing. If we draw in the tangents to the curve, you will notice that if the gradient of the tangent is ...

, , Increasing or Decreasing Returns to Scale in the Constant Elasticity of Substitution Production Function, The Review of Economics and Statistics, Vol. 51, No. 1 (Feb., 1969), pp. 84-90 ...