*, so that:
ERU[FE] = uf = f -1[ (1 -
) (
-
[-1]) ] (28.4)
This equation has the same format as ERU[REH]. It follows that uf can be REH, and REH could be uf. However, they need not be, since, though we have used the same symbol f, in practice there can be different functions and also additional variables depending upon the FE or REH assumption. [90]