Linearly Independent/Dependent

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Homework Help Overview

The discussion revolves around the classification of linear differential equations, specifically examining the conditions under which certain equations are considered linear or non-linear. Participants explore the implications of linear dependence and independence in the context of differential equations.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants question the definitions of linearity in differential equations, particularly whether the presence of certain terms affects linearity. They discuss specific examples and the criteria for determining if an equation is linear based on the degrees of the terms involved.

Discussion Status

There is an ongoing exploration of different examples of differential equations, with some participants providing clarifications on the definitions of linear and non-linear equations. Multiple interpretations of the criteria for linearity are being examined, and guidance has been offered regarding specific cases.

Contextual Notes

Participants are navigating the definitions of linearity in the context of differential equations, with some expressing confusion over the role of coefficients and terms in determining linearity. The discussion reflects a mix of understanding and uncertainty regarding these concepts.

JosephK
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Is a linear equation y'+P(x)y=Q(x) not linear if P(x) and Q(x) are not linearly dependent function?

Does linearly dependent mean a constant multiplied by P(x) will equal Q(x)?

Thank you.
 
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JosephK said:
Is a linear equation y'+P(x)y=Q(x) not linear if P(x) and Q(x) are not linearly dependent function?

Does linearly dependent mean a constant multiplied by P(x) will equal Q(x)?

Thank you.

No.

The equation itself may or not be linear. In this case, it is because y and y' occur only as first degree. It has nothing to do with x.

An nth order linear homogeneous DE will have n linearly independent solutions. That is not the same concept. A first order DE as in your example can not have two linearly independent solutions. So if your example was y' + P(x)y = 0, and y is a solution, then any constant time y is a solution. Such solutions are linearly dependent.
 
In recognizing linear differential equations for example y'+3x^2y=x^2 I do not say this linear differential equation is linear because I can multiply x^2 by 3).

I should say because y and y' are of the first degree this equation is linear.

What about y'-y=xy^2?

Is this linear differential equation non-linear because the y to the right is not in the first degree?

What about y'-2xy=1/x*y*lny?

Is this linear differential equation not linear because the coefficient of the y to the right hand side depends on y?

Thank you.
 
JosephK said:
In recognizing linear differential equations for example y'+3x^2y=x^2 I do not say this linear differential equation is linear because I can multiply x^2 by 3).

I should say because y and y' are of the first degree this equation is linear.
Correct.
What about y'-y=xy^2?

Is this linear differential equation non-linear because the y to the right is not in the first degree?
Correct.
edit: I read this too fast. As Mark44 points out below, an equation is either linear or not. This equation is non-linear.
What about y'-2xy=1/x*y*lny?

Is this linear differential equation not linear because the coefficient of the y to the right hand side depends on y?
It's non-linear because a non-linear function of y (y * ln y) appears in the equation.
 
Last edited:
JosephK said:
In recognizing linear differential equations for example y'+3x^2y=x^2 I do not say this linear differential equation is linear because I can multiply x^2 by 3).

I should say because y and y' are of the first degree this equation is linear.
Yes. This is a linear differential equation. A linear DE is one in which y, y', y'', etc. occur to the first power. How the independent variable (x in this case) occurs doesn't enter into the description.
JosephK said:
What about y'-y=xy^2?

Is this linear differential equation non-linear because the y to the right is not in the first degree?
No. This differential equation is nonlinear for the reason you say. A differential equation is either linear or nonlinear. It makes no sense to ask if a linear DE is nonlinear. If it's nonlinear it is not a linear differential equation.
JosephK said:
What about y'-2xy=1/x*y*lny?

Is this linear differential equation not linear because the coefficient of the y to the right hand side depends on y?
Your question should be, "Is this differential equation nonlinear ..." It is nonlinear because of the lny factor on the right side.
 

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