Does Finding an Eigenvalue Guarantee an Eigenvector?

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Discussion Overview

The discussion centers on the relationship between eigenvalues and eigenvectors in linear algebra, specifically whether the existence of an eigenvalue guarantees the existence of at least one corresponding eigenvector. The scope includes theoretical aspects of linear operators and the concepts of algebraic and geometric multiplicity.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that finding at least one eigenvalue implies the existence of at least one eigenvector.
  • Others clarify the distinction between algebraic and geometric multiplicity, suggesting that while an eigenvalue may have a certain multiplicity, the number of linearly independent eigenvectors may differ.
  • A participant questions whether it is possible to have no eigenvectors for some eigenvalues, given the definitions and explanations provided.
  • Another participant asserts that every eigenvalue must have at least one corresponding eigenvector, but acknowledges that repeated eigenvalues can have fewer linearly independent eigenvectors than their algebraic multiplicity suggests.
  • One participant emphasizes that the definition of an eigenvalue inherently includes the existence of an eigenvector, and discusses the implications of linear combinations of eigenvectors.

Areas of Agreement / Disagreement

Participants generally agree that at least one eigenvector exists for every eigenvalue, but there is contention regarding the implications of multiplicity and the number of independent eigenvectors associated with repeated eigenvalues.

Contextual Notes

The discussion highlights the nuances of eigenvalue and eigenvector relationships, particularly concerning definitions and the implications of multiplicity, without resolving the complexities involved.

Who May Find This Useful

Readers interested in linear algebra, particularly those exploring eigenvalues and eigenvectors, as well as the concepts of algebraic and geometric multiplicity.

sjeddie
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Is at least one eigenvector guaranteed to exist given that we have found at least one eigenvalue? So, for example, given that we have found an eigenvalue of multiplicity 2 of a matrix, are we guaranteed to find at least 1 eigenvector of that matrix? Why or why not?
 
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Thanks.

So it is possible to have less eigenvectors than eigenvalues (according to wiki's explanation on geometric multiplicity), so is it possible to have no eigenvectors at all for some eigenvalues?
 
Not quite. Every eigenvalue will have at least one eigenvector. It is possible, however, for a repeated eigenvalue to have less linearly independent eigenvectors (its geometric multiplicity) than the number of repetitions of that eigenvalue (its algebraic multiplicity). There will always be at least one eigenvector though.
 
thank you very much rochfor1!
 
The definition of "eigenvalue" is "[itex]\lambda[/itex] is an eigenvalue for linear operator A if and only if there exist a non-zero vector, v, such that [itex]Av= \lambda v[/itex]".

Such a vector is, of course, an eigenvector so, by definition, there exist at least one eigenvector corresponding to any eigenvalue. And, in fact, any multiple of an eigenvector or any linear combination of eigevectors corresponding to the same eigenvalue is also an eigenvector- there necessarily exist an infinite number of eigenvectors corresponding to any eigenvalue- they form a subspace.

Rochfor1 is specifically talking about the number of independent eigenvectors corresponding to each eigenvalue- the dimension of that subspace. That's the "geometric multiplicity" of that eigenvalue.
 

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