12For example, CRRA utility is well defined only on the positive real numbers, so for a CRRA utility consumer c = 0  . In other cases, for example with exponential or quadratic cases, there is nothing to prevent consumption of − ∞ , so for those models c = − ∞ , unless there is a desire to restrict the model to positive values of consumption, in which case the c ≥ 0  constraint will be implemented through the use of (6).