# Finding eigenvalues and eigenspaces with only this info (1 Viewer)

### Users Who Are Viewing This Thread (Users: 0, Guests: 1)

#### Dist

Lets say I have a 3x3 matrix 'A' and one known eigenvalue 'z' and one known eigenvector 'x', but they don't "belong" to each other, as in Ax =/= zx
Is there a way of finding the other eigenvalues and eigenspaces of A using only this piece of information?
Thanks.

#### Simon_Tyler

I doubt it.

Just count the amount of information needed and available.
A 3x3 matrix has 9 numbers.
An eigenvalue is one number.
An eigenvector is two numbers (it's only defined upto a scale factor)
Thus in total you have 3<9 pieces of information.

Note that the full eigensystem is 3 eigenvalues and 3 eigenvectors
= 3*1 + 3*2 = 3*(1+2) = 9 pieces of information.

#### HallsofIvy

Even if the eigenvalue and eigenvector "belong" to each other, the other eigenvalues and eigenvectors could be anything. There will exist an infinite number of matrices having the given eigenvector and eigenvalue.

Or are you saying that you have a known matrix, one eigenvalue and eigenvector, and want to know if there is a simpler way of finding the other eigenvalues and eigenvectors than the usual "solve the characteristic equation"?

#### Dist

No, the matrix A is unknown as well. And I only want to find the eigenvalues and eigenspaces of A, not necessarily its eigenvectors.

#### Simon_Tyler

If you know the eigenspaces then you can choose a set of eigenvectors. So, your last statement is a little meaningless.

#### HallsofIvy

As I said before, given one eigenvalue and one eigenvector, whether they correspond or not, there exist an infinite number of matrices with those given eigenvalue and eigenvector and whatever other eigenvalues and eigenvectors you want to assign.

A single eigenvector and eigenvalue tell you nothing about other eigenvalues and eigenvectors.

#### Dist

Ok, I see, thanks.
But say we could choose arbitrary eigenvalues and eigenvectors that would go with the ones we already know. How would one go about doing that?

### The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving