Differential Equations, Discrete Systems and Control: Economic Models

In t.lw fHll of !!)!)2, Professor Dr. M. Alt.ar, chairman of tIw newly established dppartnwnt or Managenwnt. wit.h Comput.er Science at thp Homanian -American Univprsity in Bucharest (a private univprsil.y), inl.roducod in t.he curriculum a course on DiffenHltial Equations and Optimal Cont.rol, asking lIS to teach such course. It was an inter8sting challengo, since for t.Iw first tim8 wo had to t8ach such mathemaLical course for st.udents with economic background and interosts. It was a natural idea to sl.m't by looking at pconomic models which were described by differpntial equations and for which problems in (\pcision making dir! ariso. Since many or such models were r!escribed in discret.e timp, wp eleculed to elpvolop in parallel t.he theory of differential equations anel thaI, of discrete-timo systpms aur! also control theory in continuous and discrete time. Tlw jll'eSPlu book is t.he result of our tpaehing px!wripnce wit.h this courge. It is an enlargud version of t.he actllal lectuf(~s where, depending on t.he background of tho St.lI(\('Ilts, not all proofs could be given in detail. We would like to express our grat.itude to tlw Board of the Romanian - American University, personally 1. 0 the Rector, Professor Dr. Ion Smedpscu, for support, encouragement and readinpss to accept advancnd ideas in tho curriculum. fhe authors express t.heir warmest thanks 1.0 Mrs. Monica Stan . Necula for tho oxcellent procC'ssing of t.he manuscript.

Table of Contents:
1 Linear and Affine Differential Equations. Equations with Separated Variables.- 1.1 Differential Equations Modelling Growth Processes.- 1.2 Linear Differential Equations.- 1.3 Linear Affine Differential Equations.- 1.4 Simplest Models of Price Evolution in a Market Economy.- 1.5 Discrete — Time Models for Price Evolution.- 1.6 Simplest Models for Economic Growth.- 1.7 Discrete — Time Models for Economic Growth.- 1.8 Production Functions.- 1.9 Equations with Separated Variables.- 1.10 Notes and References.- 2 Linear Differential Equations with Constant Coefficients.- 2.1 Second Order Differential Equations with Constant Coefficients.- 2.2 Discrete — Time Second Order Linear Equations.- 2.3 Price Evolution in the Presence of Inventories.- 2.4 Economic Growth Models.- 2.5 Second Order Linear Affine Equations.- 2.6 The Phillips Model with Several Types of Autonomous Investment.- 2.7 Higher Order Linear Differential Equations with Constant Coefficients.- 2.8 Discrete — Time Linear Affine Equations.- 2.9 The Samuelson — Hicks Model for Economic Growth.- 2.10 Notes and References.- 3 Linear Systems with Constant Coefficients.- 3.1 General Form of Solutions.- 3.2 Matrix Exponential.- 3.3 Linear Affine Systems.- 3.4 Economic Models.- 3.5 Leontieff — type Models.- 3.6 Phase — Portrait for Second Order Linear Systems with Constant Coefficients.- 3.7 Notes and References.- 4 General Theory of Nonlinear Systems. Stability.- 4.1 Existence and Uniqueness Theorem for the Initial Value Problem.- 4.2 Equilibria. Stability. Continuous Time.- 4.3 Stability. Discrete Time.- 4.4 Discrete—Time Logistic Equation.- 4.5 Stable Polynomials.- 4.6 Some Properties of Matrices that occur in Economic Models.- 4.7 Notes and References.- 5 Numerical Solution of Differential Equations.-5.1 Euler Method.- 5.2 Richardson Extrapolation.- 5.3 Predictor — Corrector Methods.- 5.4 Numerical Quadrature.- 5.5 Adams Type Methods.- 5.6 Stiff Systems.- 5.7 Some Applications of Differential Equations in Numerical Analysis and Optimization.- 5.8 Notes and References.- 6 Control Systems. Stabilization of Linear Systems.- 6.1 Stabilization Problem. Stabilization by Linear State Feed-Back.- 6.2 Stabilization of Linear Systems by Using a Controller.- 6.3 Stabilization in an Economic Growth Model.- 6.4 A Monetary Policy Model.- 6.5 Stabilization of Discrete—Time Systems.- 6.6 A Discrete—Time Monetary Policy Model.- 6.7 Notes and References.- 7 Optimal Stabilization.- 7.1 Linear—Quadratic Optimization on Infinite Horizon. Continuous Time.- 7.2 Application to a Price Model.- 7.3 Optimal Stabilization in Discrete Time.- 7.4 Optimal Stabilization in a Discrete—Time Model of Price Evolution.- 7.5 Notes and References.- 8 Linear—Quadratic Optimization on Finite Horizon.- 8.1 Continuous Time.- 8.2 Applications.- 8.3 Discrete Time.- 8.4 Applications in Discrete Time.- 8.5 A Tracking Problem.- 8.6 A Simple Differential Game.- 8.7 Notes and References.- 9 Some Unconstrained Dynamic Optimization Problems.- 9.1 The Simplest Problem of the Calculus of Variations.- 9.2 A Macroeconomic Growth Model.- 9.3 A Discrete — Time Variational Problem.- 9.4 An Application.- 9.5 Unrestricted Optimal Control Problem in Discrete Time.- 9.6 An Application.- 9.7 Optimization with Linear Dynamics and Linear Cost. Continuous Time.- 9.8 Some Microeconomic Models.- 9.9 Optimization with Linear Dynamics and Linear Cost. Discrete Time.- 9.10 Applications.- 9.11 Notes and References.- 10 General Problem of Optimal Control.- 10.1 Problem Statement. General Theorems.- 10.2 Optimum CapitalAccumulation under the Minimum Time Objective.- 10.3 Reduction of Problems with Free Initial and Final Time to Problems on Fixed Horizon.- 10.4 An Abstract Multiplier Rule.- 10.5 Proof of Theorem 10.1.- 10.6 Notes and References.- References.