Một trong những tính năng hấp dẫn nhất của lý thuyết thu nhập cố định, kể từ khi xây dựng ban đầu do M. Friedman, là một lời giải thích tiềm năng của lý do tại sao tiêu thụ thường ít biến động hơn so với thu nhập hiện tại, ngay cả trong các mô hình sách giáo khoa của Keynes đơn giản, một | INVESTMENT 71 exogenous variables but rather verifies a property that endogenous variables should display under certain theoretical assumptions. As regards revenues the assumption leading to the conclusion that investment and average q should be strictly related may be interpreted supposing that the firm produces under constant returns to scale and behaves in perfectly competitive fashion. As regards adjustment costs the assumption is that they pertain to proportional increases of the firm s size rather than to absolute investment flows. A larger firm bears smaller costs to undertake a given amount of investment and the whole optimal investment program may be scaled upwards or downwards if doubling the size of the firms yields the same unit investment costs for twice-as-large investment flows that is if the adjustment cost function has constant returns to scale and G I K g I K K. The realism of these like any other assumptions is debatable of course. They do imply that different initial sizes of the firm simply yield a proportionally rescaled optimal investment program. As always under constant returns to scale and perfectly competitive conditions the firm does not have an optimal size and in fact does not quite have a well-defined identity. In more general models the value of the firm is less intimately linked to its capital stock and therefore may vary independently of optimal investment flows. . A Dynamic IS-LM Model We are now ready to apply the economic insights and technical tools introduced in the previous sections to study an explicitly macroeconomic and explicitly dynamic modeling framework. Specifically we discuss a simplified version of the dynamic IS-LM model of Blanchard 1981 capturing the interactions between forward-looking prices of financial assets and output and highlighting the role of expectations in determining through investment macroeconomic outcomes and the effects of monetary and fiscal policies. As in the static version of the IS-LM .
