Eigenvalues and Eigenvectors of 3x3 matricies

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Discussion Overview

The discussion revolves around finding the eigenvalues and eigenvectors of 3x3 matrices, specifically addressing challenges related to manipulating the characteristic polynomial derived from the determinant of (A - mI). Participants explore various methods and strategies for solving cubic equations that arise in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses difficulty in manipulating the characteristic polynomial to find eigenvalues.
  • Another suggests solving the cubic equation by guessing a solution, dividing out a factor, or using a general formula for cubic equations.
  • A different participant mentions the rational root theorem as a method to find potential rational roots of the polynomial.
  • Another proposes diagonalizing the matrix or using similarity transformations as alternative approaches, though they note it may not simplify the problem.
  • One participant suggests factoring the determinant into linear factors as a potential method.
  • Another highlights properties of eigenvalues, stating that the trace of the matrix equals the sum of the eigenvalues and the determinant equals their product.
  • A repeated post reiterates the initial difficulty and requests examples of specific polynomial equations for further assistance.

Areas of Agreement / Disagreement

Participants present multiple competing views and methods for addressing the problem, with no consensus on a single approach or solution. The discussion remains unresolved as participants explore various strategies without agreement on the best method.

Contextual Notes

Some methods mentioned depend on the specific characteristics of the polynomial, such as the presence of rational roots or integer coefficients, which may not apply universally. The discussion does not resolve the complexities involved in solving cubic equations.

9_lulu_0
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Hello

Im trying to find the eigenvalues and eigenvectors of 3x3 matricies, but when i take the determinant of the char. eqn (A - mI), I get a really horrible polynomial and i don't know how to minipulate it to find my three eigenvalues.

Can someone please help..
Thanks
 
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There is usually nothing fancy about it, you just need some way to solve the cubic equation. Often it is possible to guess one solution, divide out a factor and solve the resulting quadratic equation. Otherwise there may be other tricks available, or you might need some general formula (the degree 3 equivalent of the quadratic "abc" formula).

Perhaps it helps if you post an example of a matrix and your result for the characteristic equation where you have doubts?
 
Cubic equations, in general, are difficult.

If the equation has integer coefficients, you can try to use the 'rational root theorem': if m/n is a a rational root of the polynomial equation, [itex]ax^n+ \cdot\cdot\cdot+ bx+ c= 0[/itex], with integer coefficients, then the denominator, n, must divide the leading coefficient, a, and the numerator, m, must divide the constant term, c. You can factor those two cofficients to find all possible rational roots, then put them into the equation to see if they work.

Of course, it is possible that a cubic equation does not have any rational roots. In that case, you would have to use "Cardano's cubic formula"

I don't see how we can say more without seeing the specific matrix or cubic equation you are talking about.
 
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Try diagonalizing the matrix... Look for similarity transformation... But it is not easier than characteristic polynomial anyway. But at least you stay with matrices instead of polynomialshence, a little bit more fun.
 
Try factoring the determinant into linear factors?
 
There's a couple of neat properties when finding eigenvalues that might help. For one, the trace of your matrix is equal to the sum of your eigenvalues. The determinant is equal to the product of the eigenvalues as well.
 
9_lulu_0 said:
Hello

Im trying to find the eigenvalues and eigenvectors of 3x3 matricies, but when i take the determinant of the char. eqn (A - mI), I get a really horrible polynomial and i don't know how to minipulate it to find my three eigenvalues.

Can someone please help..
Thanks
So give us some examples of polynomial equations you are having troulbe with.
 

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