# 10: Linear Systems of Differential Equations

- Page ID
- 30786

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IN THIS CHAPTER we consider systems of differential equations involving more than one unknown function. Such systems arise in many physical applications.SECTION 10.1 presents examples of physical situations that lead to systems of differential equations. SECTION 10.2 discusses linear systems of differential equations. SECTION 10.3 deals with the basic theory of homogeneous linear systems. SECTIONS 10.4, 10.5, AND 10.6 present the theory of constant coefficient homogeneous systems. SECTION 10.7 presents the method of variation of parameters for nonhomogeneous linear systems.

- 10.1: Introduction to Systems of Differential Equations
- Many physical situations are modeled by systems of n differential equations in n unknown functions, where n≥2 . This section presents examples of physical situations that lead to systems of differential equations.

- 10.2: Linear Systems of Differential Equations
- A first order system of differential equations are introduced.

- 10.3: Basic Theory of Homogeneous Linear Systems
- In this section we consider homogeneous linear systems y′=A(t)y, where A=A(t) is a continuous n×n matrix function on an interval (a,b). The theory of linear homogeneous systems has much in common with the theory of linear homogeneous scalar equations.

- 10.4: Constant Coefficient Homogeneous Systems I
- We’ll now begin our study of the homogeneous system y′=Ay, where A is an n×n constant matrix. . In this section we assume that all the eigenvalues of A are real and that A has a set of n linearly independent eigenvectors. In the next two sections we consider the cases where some of the eigenvalues of A are complex, or where A does not have n linearly independent eigenvectors.

- 10.5: Constant Coefficient Homogeneous Systems II
- In this section we consider the case where A has n real eigenvalues, but does not have n linearly independent eigenvectors. It is shown in linear algebra that this occurs if and only if A has at least one eigenvalue of multiplicity r>1 such that the associated eigenspace has dimension less than r . In this case A is said to be defective. We will restrict our attention to some commonly occurring special cases.

- 10.6: Constant Coefficient Homogeneous Systems III
- We now consider the system y′=Ay , where A has a complex eigenvalue λ=α+iβ with β≠0 . We continue to assume that A has real entries, so the characteristic polynomial of A has real coefficients. This implies that λ=α−iβ is also an eigenvalue of A .

- 10.7: Variation of Parameters for Nonhomogeneous Linear Systems
- We now discuss an extension of the method of variation of parameters to linear nonhomogeneous systems. This method will produce a particular solution of a nonhomogenous system y′=A(t)y+f(t) provided that we know a fundamental matrix for the complementary system.