Explain briefly, the meaning of this theorem about producer theory. THEOREM 3.6 Integrability for Cost Functions A continuously differentiable function x(w, y) mapping R" , x R+ into R" is the con- ditional input demand function generated by some strictly increasing, quasiconcave production function if and only if it is homogeneous of degree zero in w, its substitut tion matrix, whose ijth entry is axi(w, y) /wj, is symmetric and negative semidefinite, and w . x(w, y) is strictly increasing in y