# What exactly is linear dependency? 2nd and 3rd Order Diff EQ

• I
• sparkie
In summary, linear dependence or independence in the context of nth order constant coefficient non-homogeneous ODEs refers to the linear independence or dependence of solutions to the differential equation. This concept is important in finding the general solution to a differential equation, and can be determined by examining the relationship between the solutions. The method of annihilators is one technique that can be used to solve nonhomogeneous ODEs.
sparkie
We are studying 2nd and 3rd order differential equations in class, and have touched on superposition and were talking about an equation being linearly dependent or independent. I received some good explanations from tutors about this, using vectors as examples, but I'm still a bit unclear on the concept. I'm really trying to think of a way to make the question more specific, but I don't know that I have a good enough grasp conceptually to do that. I do know that I have this one question nagging me, though:

What does linear dependence or independence mean to an nth order constant coefficient non-homogeneous ODE?

talk2dream
sparkie said:
We are studying 2nd and 3rd order differential equations in class, and have touched on superposition and were talking about an equation being linearly dependent or independent. I received some good explanations from tutors about this, using vectors as examples, but I'm still a bit unclear on the concept. I'm really trying to think of a way to make the question more specific, but I don't know that I have a good enough grasp conceptually to do that. I do know that I have this one question nagging me, though:

What does linear dependence or independence mean to an nth order constant coefficient non-homogeneous ODE?
It doesn't make much sense to talk about a single equation being linearly dependent or linearly independent. It makes more sense to talk about solutions of differential equations where the solutions are linearly independent. Many of the ideas from linearly independent/dependent vectors come into play here.

With two vectors or two functions, it's easy to determine whether they are linearly independent or linearly dependent. Dependent vectors or functions are multiples of each other. With three vectors or three functions, it's not as obvious, but no one of them can be a linear combination of the others. For example, the three functions ##f_1 = \sin^2(x), f_2(x) = \cos^2(x), f_3(x) = 7## are linearly dependent, since the third function is a linear combination of the other two. Specifically, ##f_3(x) = 7f_1(x) + 7f_2(x)##.

Obtaining a set of linearly independent solutions is important to being able to specify the general solution of a differential equation, whether it's homogeneous or not. It's just a bit trickier if you're working with a nonhomogeneous diff. equation, because sometimes a solution of the homogeneous problem won't also be a solution of the nonhomogeneous problem. For example, if the diff. equation if ##y'' + y = \sin(x)##, it turns out that ##y = \sin(x)## is one solution (another is ##y = \cos(x)##) of the homogeneous problem y'' + y = 0, but it won't also be a solution of the equation ##y'' + y = \sin(x)##.

One technique that can be used to solve nonhomogeneous, linear, constant coefficient diff. equations is the method of annihilators, in which you convert a given nonhomogeneous problem into a homogeneous problem of higher order. There's an Insights article or two about this technique (written by me). You should probably be able to find them fairly easily if you search for "annihilator" among these articles.

## What exactly is linear dependency?

Linear dependency refers to the relationship between two or more variables where one variable can be expressed as a linear combination of the other variables. In other words, there is a constant ratio between the different variables.

## What are 2nd and 3rd Order Diff EQ?

2nd and 3rd Order Diff EQ stands for 2nd and 3rd Order Differential Equations. These are types of mathematical equations that involve derivatives of a function. They are used to model various physical phenomena and can be solved using various methods such as separation of variables, substitution, and integration.

## How do you determine if a set of equations is linearly dependent or independent?

To determine if a set of equations is linearly dependent or independent, you can use the determinant method. If the determinant of the coefficient matrix is equal to zero, then the equations are linearly dependent. If the determinant is not equal to zero, then the equations are linearly independent.

## What is the difference between linear and non-linear dependency?

The main difference between linear and non-linear dependency is the type of relationship between the variables. In linear dependency, the relationship is proportional and can be expressed as a straight line on a graph. In non-linear dependency, the relationship is not proportional and cannot be represented by a straight line on a graph.

## How are 2nd and 3rd Order Diff EQ used in scientific research?

2nd and 3rd Order Diff EQ are commonly used in scientific research to model and analyze various physical phenomena such as heat transfer, fluid dynamics, and population growth. They can also be used to predict future outcomes and make informed decisions based on the behavior of the variables involved.

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