# Differential Equations : Definitions & Terminology

## Example Questions

### Example Question #1 : Definitions & Terminology

State the order of the given differential equation and determine if it is linear or nonlinear.

Fourth ordered, linear

Third ordered, nonlinear

Second ordered, nonlinear

Second ordered, linear

Third ordered, linear

Third ordered, linear

Explanation:

This problem contains two questions that need to be solved for: order of the differential equation and whether it is linear or nonlinear.

To determine the order of the differential equation, look for the highest derivative in the equation.

For this particular function recall that,

therefore the highest derivative is three which makes the equation a third ordered differential equation.

The second part of this problem is to determine if the equation is linear or nonlinear. For a differential equation to be linear two characteristics must hold true:

1. The dependent variable  and all its derivatives have a power involving one.

2. The coefficients depend on the independent variable .

Looking at the given function,

it is seen that all the variable  and all its derivatives have a power involving one and all the coefficients depend on  therefore, this differential equation is linear.

To answer this problem completely, the differential equation is a linear, third ordered equation.

### Example Question #1 : Definitions & Terminology

Which of the following three equations enjoy both local existence and uniqueness of solutions for any initial conditions?

Explanation:

By the cauchy-peano theorem, for , as long as  is continuous on a closed rectangle around our starting point, we have local existence. All three functions are continuous everywhere, so they enjoy local existence at every starting point.

We can show that the solutions to differential equations are unique by showing that  is Lipschitz continuous in y. If  is continuous, then this will suffice to show the Lipschitz continuity.

Note that the first and third equations are continuous for all y and t, but that the second is not continuous when . More concretely, when , both the equation  and the equation  would satisfy the differential equation.

### Example Question #3 : Definitions & Terminology

State the order of the given differential equation and determine if it is linear or nonlinear.

Third ordered, nonlinear

Second ordered, nonlinear

Fourth ordered, linear

Third ordered, linear

Second ordered, linear

Third ordered, linear

Explanation:

This problem contains two questions that need to be solved for: order of the differential equation and whether it is linear or nonlinear.

To determine the order of the differential equation, look for the highest derivative in the equation.

For this particular function recall that,

therefore the highest derivative is three which makes the equation a third ordered differential equation.

The second part of this problem is to determine if the equation is linear or nonlinear. For a differential equation to be linear two characteristics must hold true:

1. The dependent variable  and all its derivatives have a power involving one.

2. The coefficients depend on the independent variable .

Looking at the given function,

it is seen that all the variable  and all its derivatives have a power involving one and all the coefficients depend on  therefore, this differential equation is linear.

To answer this problem completely, the differential equation is a linear, third ordered equation.

### Example Question #1 : Definitions & Terminology

Which of the following definitions describe an autonomous differential equation.

A differential equation that does not depend explicitly on the independent variable of the equation; usually denoted  or .

A differential equation that does not depend explicitly on the dependent variable of the equation; usually denoted .

A differential equation that has Eigen Values of 0.

A differential equation that models growth exponentially.

A differential equation that does not depend explicitly on the independent variable of the equation; usually denoted  or .

Explanation:

By definition, an autonomous differential equation does not depend explicitly on the independent variable. An autonomous differential equation will take the form

### Example Question #5 : Definitions & Terminology

State the order of the given differential equation and determine if it is linear or nonlinear.

Third ordered, linear

Second ordered, nonlinear

Third ordered, nonlinear

Second ordered, linear

Fourth ordered, linear

Third ordered, linear

Explanation:

This problem contains two questions that need to be solved for: order of the differential equation and whether it is linear or nonlinear.

To determine the order of the differential equation, look for the highest derivative in the equation.

For this particular function recall that,

therefore the highest derivative is three which makes the equation a third ordered differential equation.

The second part of this problem is to determine if the equation is linear or nonlinear. For a differential equation to be linear two characteristics must hold true:

1. The dependent variable  and all its derivatives have a power involving one.

2. The coefficients depend on the independent variable .

Looking at the given function,

it is seen that all the variable  and all its derivatives have a power involving one and all the coefficients depend on  therefore, this differential equation is linear.

To answer this problem completely, the differential equation is a linear, third ordered equation.

### Example Question #1 : Definitions & Terminology

Find Order and Linearity of the following differential equation

Third Order - Linear

Second Order - NonLinear

Third Order - NonLinear

Second Order - Linear