If I have four input factors (a, b, c, b) and I want to construct a nested CES production function such that (a, b) are substitutes, (c, d) are substitutes and [(a, b), (c, d)] are complements, I.e. a, b together are complements of c, d together. How would such a production function look like?
I saw some people include a Cobb Douglas there to simplify the function. So, under what situations do people use Cobb Douglas in a nested CES? Cause the factor price of a here in my model will decrease asymptotically to zero, if use the classic CES, b will be completely substituted, which I don’t want.