Calculate Cosine of a Matrix: Solutions to Systems

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    Cosine Matrix
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Discussion Overview

The discussion revolves around the calculation of the cosine of a matrix and its application to solving systems of equations. Participants explore the definition of the cosine of a matrix and its relation to differential equations, particularly in the context of numeric functions and series definitions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant presents the definition of the cosine of a matrix as a series expansion and expresses uncertainty about its application to solving systems.
  • Another participant confirms the definition and suggests that the cosine behaves like a regular cosine, implying that understanding its properties may be sufficient for the problem at hand.
  • A different perspective is introduced, proposing two approaches: one involving the general solution to a second-order differential equation and the other involving term-by-term differentiation of the series definition of cosine.
  • There is a request for clarification on how the cosine relates to the problem, indicating a lack of understanding of the connection between the matrix cosine and the solutions to the systems.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach to apply the cosine of a matrix to the problem. Multiple perspectives and methods are presented, highlighting differing interpretations of the relationship between the cosine function and the systems being solved.

Contextual Notes

Some assumptions about prior knowledge of differential equations and series expansions are present, which may limit the discussion for participants unfamiliar with these concepts. The discussion also reflects uncertainty regarding the application of the matrix cosine in practical scenarios.

jessicamorgan
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Please help, I know the definition of a cosine of a matrix is cos(a) = I-1/2!A^2+1/4!A^4-...
But I am unsure how this would help me find solutions to these systems.
 
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jessicamorgan said:
Please help, I know the definition of a cosine of a matrix is cos(a) = I-1/2!A^2+1/4!A^4-...
But I am unsure how this would help me find solutions to these systems.

Hi jessicamorgan! Welcome to MHB! (Smile)

That is indeed the definition of the cosine of a matrix.
We won't need it other than knowing it exists and behaves like a regular cosine.

Suppose we solve the equation for the 1-dimensional case, when the cosine is really a regular cosine.
What would the solution be?
 
I like Serena said:
Hi jessicamorgan! Welcome to MHB! (Smile)

That is indeed the definition of the cosine of a matrix.
We won't need it other than knowing it exists and behaves like a regular cosine.

Suppose we solve the equation for the 1-dimensional case, when the cosine is really a regular cosine.
What would the solution be?

Hi, thanks for replying.
I'm unsure of the question, I don't understand how to solve or what the cosine has to do with it. :/
 
It looks to me like there are two different ways to look at this problem.
1) That you have already learned that, for numeric functions, the general solution to [math]y''= -A^2y[/math] is $$y(x)= C cos(\sqrt{A}x)+ Dsin(\sqrt{A}x)$$ and are asked to use the series definitions to write that same solution for A a matrix.

2) That you have already learned that you can differentiate such a series "term by term" so that, given [math]y= cos(Ax)= I-1/2!(Ax)^2+1/4!(Ax)^4-...[/math] you have [math]y'= -A(Ax)+ 1/3!A(Ax)^3- ...= -A(Ax- 1/3! (Ax)^3+ ...)[/math] and then [math]y''= -A(A- 1/2!(Ax)^2+ ...)= -A^2y[/math].
 

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