# What Are Eigenvectors and Eigenvalues? - Comments

• Insights

QuantumQuest
Gold Member
Great job Mark!

RJLiberator
Gold Member
Excellent information, Mark44!

Mentor
Great job Mark!
Excellent information, Mark44!
Thanks! My Insights article isn't anything groundbreaking -- just about every linear algebra text will cover this topic. My intent was to write something for this site that was short and sweet, on why we care about eigenvectors and eigenvalues, and how we find them in a couple of examples.

RJLiberator
Dr. Courtney
Gold Member
2020 Award
I learned about eigenvalues and eigenvectors in quantum mechanics first, so I can't help but think of wavefunctions and energy levels of some Hamiltonian.

Other cases are (to me) just different analogs to wave functions and energy levels.

Nice article. I eventually took linear algebra and learned it the way you are presenting it, but I was a senior in college taking the course by correspondence.

So the red/blue arrows on the image are eigenvectors?

Mentor
So the red/blue arrows on the image are eigenvectors?
What image are you talking about? The article doesn't have any images in it.

Samy_A
Homework Helper
What image are you talking about? The article doesn't have any images in it.
It's a mystery challenging the basic foundations of Physics: he seems to refer to the image mentioned in the post following his own post.
If that is the case, the blue and violet arrows are the eigenvectors, not the red.

S.G. Janssens
What image are you talking about? The article doesn't have any images in it.
Could it be a reference to Mona Lisa at the top of the Insight?

WWGD
Gold Member
It's a mystery challenging the basic foundations of Physics: he seems to refer to the image mentioned in the post following his own post.
If that is the case, the blue and violet arrows are the eigenvectors, not the red.
How about asking Harouki to write an insight on that one -- time travel??

Samy_A
i do not understand how det(A-lambda(I))=0
since x is not a square matrix we cannot write det((A-lambda(I))*x)=det(A-lambda(I))*det(x)

Samy_A
Homework Helper
i do not understand how det(A-lambda(I))=0
since x is not a square matrix we cannot write det((A-lambda(I))*x)=det(A-lambda(I))*det(x)
Correct, you can't write that. Note that Mark44 doesn't write ##|A – \lambda I||\vec x| = 0##. He correctly writes ##|A – \lambda I|=0##.

In general, if for a square matrix ##B## there exists a non 0 vector ##\vec x## satisfying ##B\vec x=\vec 0 ##, then the determinant of ##B## must be 0.
That's how ## (A – \lambda I)\vec{x} = \vec{0}## implies ##|A – \lambda I|=0##.

Correct, you can't write that. Note that Mark44 doesn't write ##|A – \lambda I||\vec x| = 0##. He correctly writes ##|A – \lambda I|=0##.

In general, if for a square matrix ##B## there exists a non 0 vector ##\vec x## satisfying ##B\vec x=\vec 0 ##, then the determinant of ##B## must be 0.
That's how ## (A – \lambda I)\vec{x} = \vec{0}## implies ##|A – \lambda I|=0##.
but howw???
ah i got it just as i was writing this.
let 'x' be non a zero vector and let det(A) ≠ 0 .
then premultiplying with A(inverse) we get
(A(inverse)*A)*x=A(inverse)*0
am i right???
i'm sorry that i don't know how to use latex.

Samy_A
Homework Helper
but howw???
ah i got it just as i was writing this.
let 'x' be non a zero vector and let det(A) ≠ 0 .
then premultiplying with A(inverse) we get
(A(inverse)*A)*x=A(inverse)*0
am i right???
i'm sorry that i don't know how to use latex.
Yes, that's basically it.

Nice insight !!!
If you like it, I have an exemple of application for your post to euclidean geometry. You could explain how eigenvalues and eigenvectors are helpfull in order to carry out a full description of isometries in dimension 3, and conclude that they are rotations, reflections, and the composition of a rotation and a reflection about the orthogonal plane to the the axis of rotation.

Mentor
Nice insight !!!
If you like it, I have an exemple of application for your post to euclidean geometry. You could explain how eigenvalues and eigenvectors are helpfull in order to carry out a full description of isometries in dimension 3, and conclude that they are rotations, reflections, and the composition of a rotation and a reflection about the orthogonal plane to the the axis of rotation.
Thank your for the offer, but I think that I will decline. All I wanted to say in the article was a bit about what they (eigenvectors and eigenvalues) are, and a brief bit on how to find them. Adding what you suggested would go well beyond the main thrust of the article.

For how this helps the physics people, the eigen values reduce the components of a large tensor into only as many components as the order of the matrix. These reduced ones have the same effect as all the tensoral components combined. About eigenvectors, I'm not sure how it is applied.

Eigenvalues/vectors is something I've often wanted to learn more about, so I really appreciate the effort that went into writing this article Mark. The problem is that I feel like I've been shown a beautiful piece of abstract art with lots of carefully thought out splatters but the engineer in me cries out... "But what is it for?" :)

"Here is an awesome tool that is very useful to a long list of disciplines. It's called a screwdriver. To make use of it you grasp it with your hand and turn it. The end." Nooooo! Don't stop there - I don't have any insight yet into why this tool is so useful, nor intuition into the types of problems I might encounter where I would be glad I had brought my trusty screwdriver with me.

I would truly love to know these things, so I hope you will consider adding some additional exposition that offers insight into why eigenstuff is so handy.

Mentor
Eigenvalues/vectors is something I've often wanted to learn more about, so I really appreciate the effort that went into writing this article Mark. The problem is that I feel like I've been shown a beautiful piece of abstract art with lots of carefully thought out splatters but the engineer in me cries out... "But what is it for?" :)
My background isn't in engineering, so I'm not aware of how eigenvalues and eigenvectors are applicable to engineering disciplines, if at all. An important application of these ideas is in diagonalizing square matrices to solve a system of differential equations. A few other applications, as listed in this Wikipedia article (https://en.wikipedia.org/wiki/Eigenvalues_and_eigenvectors) are
• Schrödinger equation in quantum mechanics
• Molecular orbitals
• Vibration analysis
• Geology and glaciology (to study the orientation of components of glacial till)

ibkev
"All we can be sure of is that det(A–λ) must be zero". How do you arrive at this?
Are you saying that ##\vec{x}## cannot be ##\vec{0}## and ##A – \lambda## may not be ##\vec{0}##.
=>##A – \lambda## does not have an inverse.
=>##det(A – \lambda)## must be 0. Is that the reasoning?

"All we can be sure of is that the determinant of IA–λI must be zero". How do you arrive at this?
Are you saying that vec{x} cannot be vec{0}. And A – lambda may not be vec{0}.

=>A – lambda does not have an inverse.
=>det|A – lambda| must be 0.
Is that the reasoning?
Sorry my LaTex was all messed up. Here is what I meant to say...

"All we can be sure of is that det(A–λ) must be zero". How do you arrive at this?
Are you saying that ##\vec{x}## cannot be ##\vec{0}## and ##A – \lambda## may not be ##\vec{0}##.
=>##A – \lambda## does not have an inverse.
=>##det(A – \lambda)## must be 0. Is that the reasoning?

Mentor
Sorry my LaTex was all messed up. Here is what I meant to say...

"All we can be sure of is that det(A–λ) must be zero". How do you arrive at this?
Are you saying that ##\vec{x}## cannot be ##\vec{0}## and ##A – \lambda## may not be ##\vec{0}##.
=>##A – \lambda## does not have an inverse.
=>##det(A – \lambda)## must be 0. Is that the reasoning?
The full quote near the beginning of the article is this:
In the last equation above, one solution would be ##\vec{x} = \vec{0}##, but we don’t allow this possibility, because an eigenvector has to be nonzero. Another solution would be ##A – λI = \vec{0}##, but because of the way matrix multiplication is defined, a matrix times a vector can result in zero even if neither the matrix nor the vector are zero. All we can be sure of is that the determinant of |A – λI| must be zero.
The "last equation" in the quoted text above is ##(A - \lambda I)\vec{x} = \vec{0}##. My statement about |A – λI| being zero doesn't follow from ##A - \lambda I## not being invertible; it's really the other way around (i.e., since |A – λI| = 0, then A – λI doesn't have an inverse.

Note that what you wrote, A – λ, isn't correct. Subtracting a scalar ( λ) from a matrix is not defined.

a good explanation ,but .( "In other words, when a matrix A multiplies an eigenvector, the result is a vector in the same (or possibly opposite) direction,") please explain the statement