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# Differential Equations

This course is currently under construction. The target release date for this course is March.
Master a variety of techniques for solving equations that arise when using calculus to model real-world situations.

## Content

### First-Order Differential Equations

• Solve first-order differential equations using integrating factors.
• Identify and apply substitutions to convert first-order differential equations into forms that permit known solution techniques including separation of variables and integrating factors.
• Solve special first-order differential equations including exact equations and instances of Bernoulli’s equation.
• Reason about the solutions of differential equations using the existence and uniqueness theorem.
• Leverage qualitative techniques such as phase lines, equilibrium solutions, and bifurcation diagrams to infer properties of solutions of first-order differential equations.
• Construct and solve first-order differential equation models in a variety of real-world modeling contexts including population growth, temperature cooling, mechanics, and more.

### Second-Order Differential Equations

• Use reduction of order to reduce second-order differential equations to first-order differential equations for which solution techniques are known.
• Use the characteristic equation in conjunction with the superposition principle to solve homogeneous linear second-order differential equations.
• Interpret the right-hand side of an inhomogeneous differential equation as a forcing function, and use the method of undetermined coefficients to find particular solutions for various types of forcing functions.
• Use variation of parameters as a general technique for finding a particular solution to an inhomogeneous second-order differential equation.
• Solve instances of the Cauchy-Euler equation.
• Construct and solve second-order differential equation models in a variety of real-world modeling contexts including oscillators and vibrating systems.
• Extend second-order solution techniques to Nth-order linear differential equations.

### Systems of Linear Differential Equations

• Solve homogeneous systems of linear differential equations by computing eigenvalues and eigenvectors.
• Express higher-order differential equations as first-order systems.
• Extend qualitative techniques and variation of parameters to systems of linear differential equations.
• Construct and solve systems of differential equations in a variety of real-world modeling contexts including predator-prey populations and the motion of a mass on a spring.

### Alternative Solution Techniques

• Compute Laplace transforms and use them to solve differential equations.
• Find the recurrence relation that generates a power series solution for a given differential equation.
• Employ numerical techniques including Euler’s method and the Runge-Kutta method to estimate solutions to initial value problems.

• Use perturbation theory to find approximate solutions to differential equations starting from exact solutions in simpler cases.
• Construct and solve partial differential equations in a variety of real-world modeling contexts including heat diffusion, waves, and vibrating strings.
1.
First-Order Differential Equations
31 topics
1.1. First-Order Linear Differential Equations
 1.1.1. Introduction to First-Order Linear Differential Equations 1.1.2. The Linearity Principle for First-Order Linear Equations 1.1.3. Steady-State Solutions 1.1.4. Solving First-Order Linear ODEs Using Integrating Factors 1.1.5. Modeling With First-Order Linear Differential Equations
1.2. Solving Differential Equations by Substitution
 1.2.1. Homogeneous Functions 1.2.2. Homogeneous First-Order Differential Equations 1.2.3. Solving Differential Equations Using a Linear Substitution 1.2.4. Solving Differential Equations Using a Linear Substitution and Factoring 1.2.5. Reducing ODEs to First-Order Linear Using a Substitution
1.3. Special First-Order Differential Equations
 1.3.1. Exact Differential Equations 1.3.2. Bernoulli's Differential Equation
1.4. Existence, Uniqueness, and Intervals of Validity
 1.4.1. Intervals of Validity of Differential Equations 1.4.2. Existence of Solutions to Differential Equations 1.4.3. Uniqueness of Solutions to Differential Equations
1.5. Qualitative Techniques for Differential Equations
 1.5.1. Phase Lines 1.5.2. Classifying Equilibrium Solutions
1.6. Introduction to Bifurcation Theory
 1.6.1. One Parameter Families of Solutions 1.6.2. Bifurcation Diagrams 1.6.3. Bifurcations of Equilibrium Points
1.7. Applications of First-Order Differential Equations
 1.7.1. Restricted Growth Models With Differential Equations 1.7.2. Qualitative Analysis of Restricted Growth and Decay Models 1.7.3. Newton's Law of Cooling 1.7.4. Modified Logistic Growth Models With Differential Equations 1.7.5. Qualitative Analysis of Modified Logistic Growth Models 1.7.6. Velocity and Acceleration as Functions of Displacement 1.7.7. Determining Characteristics of Moving Objects Expressed as Functions of Displacement 1.7.8. Falling Body Problems 1.7.9. Dilution Problems 1.7.10. Electrical Circuits 1.7.11. Orthogonal Trajectories
2.
Second-Order Differential Equations
28 topics
2.8. Introduction to Homogeneous Linear ODEs
 2.8.1. Linear Differential Operators 2.8.2. Introduction to Linear Differential Equations 2.8.3. The Superposition Principle 2.8.4. Reduction of Order 2.8.5. The Wronskian and Linear Independence 2.8.6. General Solutions of Homogeneous Linear ODEs
2.9. Second-Order Homogeneous ODEs with Constant Coefficients
 2.9.1. Second-Order Homogeneous ODEs: Characteristic Equations With Distinct Real Roots 2.9.2. Second-Order Homogeneous ODEs: Characteristic Equations With Repeated Roots 2.9.3. Second-Order Homogeneous ODEs: Characteristic Equations With Complex Roots 2.9.4. Second-Order Homogeneous ODEs: Initial Value Problems
2.10. Second-Order Inhomogeneous ODEs with Constant Coefficients
 2.10.1. Second-Order ODEs With Polynomial Forcing 2.10.2. Second-Order ODEs With Exponential Forcing 2.10.3. Second-Order ODEs With Sinusoidal Forcing 2.10.4. The Method of Variation of Parameters
2.11. The Cauchy-Euler Equation
 2.11.1. The Cauchy-Euler Equation: Characteristic Equations With Distinct Real Roots 2.11.2. The Cauchy-Euler Equation: Characteristic Equations With Repeated Roots 2.11.3. The Cauchy-Euler Equation: Characteristic Equations With Complex Roots 2.11.4. The Cauchy-Euler Equation With Forcing
2.12. Special Second-Order Linear ODEs
 2.12.1. Airy's Differential Equation 2.12.2. Bessel's Differential Equation 2.12.3. Chebyshev Differential Equation 2.12.4. Hermite Differential Equation 2.12.5. Laguerre Differential Equation 2.12.6. Legendre Differential Equation
2.13. Higher-Order Linear ODEs
 2.13.1. Introduction to Nth-Order Linear ODEs 2.13.2. Nth-Order Linear Homogeneous Differential Equations 2.13.3. Nth-Order Linear Inhomogeneous Differential Equations 2.13.4. Variation of Parameters With Nth-Order ODEs
3.
Modeling With Second-Order Differential Equations
8 topics
3.14. Mechanical Vibrations
 3.14.1. Simple Harmonic Oscillators 3.14.2. Damped Oscillators 3.14.3. Forced Oscillators 3.14.4. Steady-State Behavior for Vibrating Systems 3.14.5. Resonance in Vibrating Systems
3.15. Further Applications of Second-Order ODEs
 3.15.1. Electrical Circuit Problems 3.15.2. Buoyancy Problems 3.15.3. Classifying Solutions
4.
Laplace Transforms
12 topics
4.16. Laplace Transforms
 4.16.1. The Dirac Delta Function 4.16.2. The Unit Step Function 4.16.3. Laplace Transforms 4.16.4. Calculating Laplace Transforms Using Tables 4.16.5. Laplace Transforms of Derivatives 4.16.6. Inverse Laplace Transforms 4.16.7. Convolutions
4.17. Solving Linear ODEs Using Laplace Transforms
 4.17.1. Solving First-Order ODEs Using Laplace Transforms 4.17.2. Solving Second-Order ODEs Using Laplace Transforms 4.17.3. Solving Nth-Order ODEs Using Laplace Transforms 4.17.4. Solving ODEs With Delta Forcing Using Laplace Transforms 4.17.5. Convolutions and Delta Forcing
5.
Systems of Differential Equations
22 topics
5.18. Systems of Linear Differential Equations
 5.18.1. Introduction to Systems of Linear Differential Equations 5.18.2. Expressing Second-Order and Third-Order Homogeneous ODEs as First-Order Systems 5.18.3. Expressing Second-Order and Third-Order Inhomogeneous ODEs as First-Order Systems 5.18.4. Phase Planes and Phase Portraits 5.18.5. The Linearity Principle for Systems of Linear ODEs
5.19. Homogeneous Systems of Linear ODEs
 5.19.1. Solving Decoupled Systems of Linear ODEs 5.19.2. Solving Systems of Linear ODEs With Real Distinct Eigenvalues 5.19.3. Systems of Linear ODEs: Initial Value Problems 5.19.4. Solving Systems of Linear ODEs With Repeated Eigenvalues 5.19.5. Solving Systems of Linear ODEs With Complex Eigenvalues 5.19.6. Solving Homogeneous Systems of ODEs Using Laplace Transforms
5.20. Phase Portraits for Systems of Linear ODEs
 5.20.1. Phase Portraits for Decoupled Linear Systems 5.20.2. Phase Portraits for Linear Systems With Real Distinct Eigenvalues 5.20.3. Phase Portraits for Linear Systems With Repeated Eigenvalues 5.20.4. Phase Portraits for Linear Systems With Complex Eigenvalues
5.21. Inhomogeneous Systems of Linear ODEs
 5.21.1. Introduction to Inhomogeneous Systems of Linear ODEs 5.21.2. Solving Inhomogeneous Systems of Linear ODEs Using Variation of Parameters 5.21.3. Solving Inhomogeneous Systems of Linear ODEs Using Laplace Transforms
5.22. Modeling With Systems of Linear ODEs
 5.22.1. The Predator-Prey Model 5.22.2. The Revised Predator-Prey Model 5.22.3. Modeling Mass-Spring Systems 5.22.4. The Lorentz Equations
6.
Boundary Value Problems
8 topics
6.23. Introduction to Boundary Value Problems
 6.23.1. Second-Order Homogeneous ODEs: Boundary Value Problems 6.23.2. Classification of Boundary Conditions 6.23.3. Eigenvalues and Eigenfunctions for Boundary Value Problems 6.23.4. Orthogonal Functions
6.24. Fourier Series
 6.24.1. Introduction to Fourier Series 6.24.2. Fourier Sine Series 6.24.3. Fourier Cosine Series 6.24.4. Convergence of Fourier Series
7.
Series and Numerical Solutions of Differential Equations
10 topics
7.25. Series Solutions of Differential Equations
 7.25.1. Taylor Series Solutions of Differential Equations 7.25.2. Power Series Solutions of Differential Equations 7.25.3. Solving Euler's Equation Using Series 7.25.4. Regular Singular Points 7.25.5. The Method of Frobenius
7.26. Numerical Solutions of Differential Equations
 7.26.1. Error and Stability in Euler's Method 7.26.2. The Modified Euler Method 7.26.3. Euler's Method for Systems of ODEs 7.26.4. The Runge-Kutta Method for First-Order Equations 7.26.5. The Runge-Kutta Method for Second-Order Equations
8.
Perturbation Methods
6 topics
8.27. Introduction to Perturbation Theory
 8.27.1. Introduction to Perturbation Theory 8.27.2. Regular Asymptotic Solutions of Polynomial Equations 8.27.3. Regular Asymptotic Solutions of Differential Equations 8.27.4. Singular Asymptotic Solutions of Polynomial Equations 8.27.5. Singular Asymptotic Solutions of Differential Equations 8.27.6. The Poincaré-Lindstedt Method
9.
Partial Differential Equations
9 topics
9.28. Partial Differential Equations
 9.28.1. Introduction to Partial Differential Equations 9.28.2. The Laplacian Operator in Cartesian Coordinates 9.28.3. The Heat Equation 9.28.4. The Wave Equation 9.28.5. Separation of Variables for Partial Differential Equations 9.28.6. Solving the Heat Equation 9.28.7. Solving the Heat Equation With Non-Zero Temperature Boundaries 9.28.8. Laplace's Equation 9.28.9. The Vibrating String Problem