Lecture Notes1 Mathematical Ecnomics
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Lecture Notes1 Mathematical Ecnomics Guoqiang TIAN Department of Economics Texas A&M University College Station, Texas 77843 ([email protected]) This version: August, 2020 1The most materials of this lecture notes are drawn from Chiang’s classic textbook Fundamental Methods of Mathematical Economics, which are used for my teaching and con- venience of my students in class. Please not distribute it to any others. Contents 1 The Nature of Mathematical Economics 1 1.1 Economics and Mathematical Economics . 1 1.2 Advantages of Mathematical Approach . 3 2 Economic Models 5 2.1 Ingredients of a Mathematical Model . 5 2.2 The Real-Number System . 5 2.3 The Concept of Sets . 6 2.4 Relations and Functions . 9 2.5 Types of Function . 11 2.6 Functions of Two or More Independent Variables . 12 2.7 Levels of Generality . 13 3 Equilibrium Analysis in Economics 15 3.1 The Meaning of Equilibrium . 15 3.2 Partial Market Equilibrium - A Linear Model . 16 3.3 Partial Market Equilibrium - A Nonlinear Model . 18 3.4 General Market Equilibrium . 19 3.5 Equilibrium in National-Income Analysis . 23 4 Linear Models and Matrix Algebra 25 4.1 Matrix and Vectors . 26 i ii CONTENTS 4.2 Matrix Operations . 29 4.3 Linear Dependance of Vectors . 32 4.4 Commutative, Associative, and Distributive Laws . 33 4.5 Identity Matrices and Null Matrices . 34 4.6 Transposes and Inverses . 36 5 Linear Models and Matrix Algebra (Continued) 41 5.1 Conditions for Nonsingularity of a Matrix . 41 5.2 Test of Nonsingularity by Use of Determinant . 43 5.3 Basic Properties of Determinants . 48 5.4 Finding the Inverse Matrix . 53 5.5 Cramer’s Rule . 58 5.6 Application to Market and National-Income Models . 63 5.7 Quadratic Forms . 66 5.8 Eigenvalues and Eigenvectors . 70 5.9 Vector Spaces . 73 6 Comparative Statics and the Concept of Derivative 81 6.1 The Nature of Comparative Statics . 81 6.2 Rate of Change and the Derivative . 82 6.3 The Derivative and the Slope of a Curve . 84 6.4 The Concept of Limit . 84 6.5 Inequality and Absolute Values . 88 6.6 Limit Theorems . 89 6.7 Continuity and Differentiability of a Function . 90 7 Rules of Differentiation and Their Use in Comparative Statics 95 7.1 Rules of Differentiation for a Function of One Variable . 95 7.2 Rules of Differentiation Involving Two or More Functions of the Same Variable . 98 CONTENTS iii 7.3 Rules of Differentiation Involving Functions of Different Vari- ables . 103 7.4 Integration (The Case of One Variable) . 107 7.5 Partial Differentiation . 110 7.6 Applications to Comparative-Static Analysis . 112 7.7 Note on Jacobian Determinants . 114 8 Comparative-Static Analysis of General-Functions 119 8.1 Differentials . 121 8.2 Total Differentials . 123 8.3 Rule of Differentials . 124 8.4 Total Derivatives . 126 8.5 Implicit Function Theorem . 130 8.6 Comparative Statics of General-Function Models . 136 8.7 Matrix Derivatives . 137 9 Optimization: Maxima and Minima of a Function of One Vari- able 139 9.1 Optimal Values and Extreme Values . 140 9.2 Existence of Extremum for Continuous Function . 141 9.3 First-Derivative Test for Relative Maximum and Minimum . 142 9.4 Second and Higher Derivatives . 145 9.5 Second-Derivative Test . 147 9.6 Taylor Series . 149 9.7 Nth-Derivative Test . 152 10 Exponential and Logarithmic Functions 153 10.1 The Nature of Exponential Functions . 153 10.2 Logarithmic Functions . 154 10.3 Derivatives of Exponential and Logarithmic Functions . 155 iv CONTENTS 11 Optimization: Maxima and Minima of a Function of Two or More Variables 159 11.1 The Differential Version of Optimization Condition . 159 11.2 Extreme Values of a Function of Two Variables . 160 11.3 Objective Functions with More than Two Variables . 167 11.4 Second-Order Conditions in Relation to Concavity and Con- vexity . 169 11.5 Economic Applications . 173 12 Optimization with Equality Constraints 177 12.1 Effects of a Constraint . 178 12.2 Finding the Stationary Values . 179 12.3 Second-Order Condition . 184 12.4 General Setup of the Problem . 186 12.5 Quasiconcavity and Quasiconvexity . 189 12.6 Utility Maximization and Consumer Demand . 195 13 Optimization with Inequality Constraints 199 13.1 Non-Linear Programming . 199 13.2 Kuhn-Tucker Conditions . 201 13.3 Economic Applications . 205 14 Differential Equations 211 14.1 Existence and Uniqueness Theorem of Solutions for Ordi- nary Differential Equations . 213 14.2 Some Common Ordinary Differential Equations with Ex- plicit Solutions . 214 14.3 Higher Order Linear Equations with Constant Coefficients . 218 14.4 System of Ordinary Differential Equations . 222 CONTENTS v 14.5 Simultaneous Differential Equations and Stability of Equi- librium . 227 14.6 The Stability of Dynamical System . 229 15 Difference Equations 231 15.1 First-order Difference Equations . 233 15.2 Second-order Difference Equation . 236 15.3 Difference Equations of Order n . 237 15.4 The Stability of nth-Order Difference Equations . 239 15.5 Difference Equations with Constant Coefficients . 240 Chapter 1 The Nature of Mathematical Economics The purpose of this course is to introduce the most fundamental aspects of the mathematical methods such as those matrix algebra, mathematical analysis, and optimization theory. 1.1 Economics and Mathematical Economics Economics is a social science that studies how to make decisions in face of scarce resources. Specifically, it studies individuals’ economic behavior and phenomena as well as how individuals, such as consumers, house- holds, firms, organizations and government agencies, make trade-off choic- es that allocate limited resources among competing uses. Mathematical economics is an approach to economic analysis, in which the economists make use of mathematical symbols in the statement of the problem and also draw upon known mathematical theorems to aid in rea- soning. Since mathematical economics is merely an approach to economic anal- 1 2 CHAPTER 1. THE NATURE OF MATHEMATICAL ECONOMICS ysis, it should not and does not differ from the nonmathematical approach to economic analysis in any fundamental way. The difference between these two approaches is that in the former, the assumptions and conclu- sions are stated in mathematical symbols rather than words and in the equations rather than sentences so that the interdependent relationship among economic variables and resulting conclusions are more rigorous and concise by using mathematical models and mathematical statistic- s/econometric methods. Also, to study, analyze, and solve practical social and economic prob- lems as well as arising your managemental skills and leadership well, I think the research methodology of "three dimensions and six natures” should be used in studying and solving realistic social economic problem- s. Three dimensions: theoretical logic, practical knowledge, and his- torical perspective; Six natures: being scientific, rigorous, realistic, pertinent, forward- looking and thought-provoking. Only by accomplishing the "three-dimensional" and “six natures", can we put forward logical, factual and historical insights, high perspicacity views and policy recommendations for the economic development of a nation and the world, as well as theoretical innovations and works that can be tested by data, practice and history. Only through the three dimen- sions, can your assessments and proposals be guaranteed to have these six natures and become a good economist. The study of economic social issues cannot simply involve real world in its experiment, so it not only requires theoretical analysis based on in- herent logical inference and, more often than not, vertical and horizontal comparisons from the larger perspective of history so as to draw experi- 1.2. ADVANTAGES OF MATHEMATICAL APPROACH 3 ence and lessons, but also needs tools of statistics and econometrics to do empirical quantitative analysis or test, the three of which are all indispens- able. When conducting economic analysis or giving policy suggestions in the realm of modern economics, the theoretical analysis often combines theory, history, and statistics, presenting not only theoretical analysis of inherent logic and comparative analysis from the historical perspective but also empirical and quantitative analysis with statistical tools for ex- amination and investigation. Indeed, in the final analysis, all knowledge is history, all science is logics, and all judgment is statistics. As such, it is not surprising that mathematics and mathematical statis- tics/econometrics are used as the basic and most important analytical tool- s in every field of economics. For those who study economics and conduct research, it is necessary to grasp enough knowledge of mathematics and mathematical statistics. Therefore, it is of great necessity to master suffi- cient mathematical knowledge if you want to learn economics well, con- duct economic research and become a good economist. 1.2 Advantages of Mathematical Approach Mathematical approach has the following advantages: (1) It makes the language more precise and the statement of as- sumptions more clear, which can deduce many unnecessary debates resulting from inaccurate verbal language. (2) It makes the analytical logic more rigorous and clearly s- tates the boundary, applicable scope and conditions for a conclusion to hold. Otherwise, the abuse of a theory may occur. (3) Mathematics can help obtain the results that cannot be eas- 4 CHAPTER 1. THE NATURE OF MATHEMATICAL ECONOMICS ily attained through intuition. (4) It helps improve and extend the existing economic theories. It is, however, noteworthy a good master of mathematics cannot guar- antee to be a good economist. It also requires fully understanding the an- alytical framework and research methodologies of economics, and having a good intuition and insight of real economic environments and econom- ic issues. The study of economics not only calls for the understanding of some terms, concepts and results from the perspective of mathematics (in- cluding geometry), but more importantly, even when those are given by mathematical language or geometric figure, we need to get to their eco- nomic meaning and the underlying profound economic thoughts and ide- als.