The Four-Equation New Keynesian Model
构建了一个包含金融中介、长短债券和信贷冲击的新凯恩斯模型，简化为四个方程，发现最优货币政策应使用短期利率应对自然率冲击，用量化宽松应对信贷市场扰动，且量化宽松能显著缓解零利率下限的约束成本。
Abstract This paper develops a New Keynesian model featuring financial intermediation, short- and long-term bonds, credit shocks, and scope for unconventional monetary policy. The log-linearized model reduces to four equations: Phillips and IS curves, as well as policy rules for the short-term interest rate and the central bank's long-bond portfolio (QE). Credit shocks and QE appear in both the IS and Phillips curves. In equilibrium, optimal monetary policy entails adjusting the short-term interest rate to offset natural rate shocks but using QE to offset credit market disruptions. Use of QE significantly mitigates the costs of a binding zero lower bound.