Nilpotent Matrices: How to Determine If Matrix is Nilpotent

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In summary: If you have a more elegant solution, then by all means go for it!In summary, the eigenvalues of an nxn matrix are found by computing the powers of the matrix, and checking to see if any of the eigenvalues are zero. If they are, then the matrix is nilpotent.
  • #1
Dragonfall
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Given an nxn matrix, how do I know whether it's nilpotent?
 
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  • #2
Compute the eigenvalues and check if they are all zero.
 
  • #3
Why is it that when you have 30 15x15 matrices, it is impossible to find out whether some chain of them will multiply out to be 0?
 
  • #4
I wonder if it's easier to just compute powers of the matrix to decide if it's nilpotent?


Anyways, for your new question, that sounds like a variant on the word problem. The direct translation of the word problem into matrices would be to tell if a product of some given matrices gives you the identity -- I have no idea if using zero instead makes a difference.
 
  • #5
well a quick and dirty solution would be checking the rank of each and every matrix and remove the invertible chains. Then you can concentrate on the nullspace vectors of those remaining. I know it really doesn't sound nice.

Singular value decomposition should be doable for 30 of them in a row.
 
  • #6
You can't eliminate the invertible ones. Consider the following set of 2x2 matrices:

[tex]
\left( \begin{array}{cc} 0 & 1 \\ 1 & 0 \end{array} \right)
\qquad \qquad
\left( \begin{array}{cc} 1 & 0 \\ 0 & 0 \end{array} \right)
[/tex]

There is a product of these matrices that gives zero... but it requires use of the invertible matrix.
 
  • #7
Damn. If I were given something like this to work on for my thesis, and it turns out to be undecidable, that'll suck.
 
  • #8
Hurkyl said:
You can't eliminate the invertible ones. Consider the following set of 2x2 matrices:

[tex]
\left( \begin{array}{cc} 0 & 1 \\ 1 & 0 \end{array} \right)
\qquad \qquad
\left( \begin{array}{cc} 1 & 0 \\ 0 & 0 \end{array} \right)
[/tex]

There is a product of these matrices that gives zero... but it requires use of the invertible matrix.

No it is not zero. Again after obtaining the nullspace vectors you can reduce the number of conditions to check...
 
  • #9
If you name the first matrix A and the second B, then ABAB is zero.
 
  • #10
I did not understand that you meant multiple products of the same matrices.

But, that is exactly what I am saying. Because a column of A is in the kernel of B. Instead of checking ABAB...AB you can check if the columns of product of the invertible ones are in the nullspace of the singular ones. If not whatever happens the product is nonzero.

Note that I said it is just a dirty way of doing it.
 

FAQ: Nilpotent Matrices: How to Determine If Matrix is Nilpotent

What is a nilpotent matrix?

A nilpotent matrix is a square matrix where the product of the matrix with itself for some number of times results in a zero matrix.

How do you determine if a matrix is nilpotent?

A matrix is nilpotent if and only if its eigenvalues are all zero. This means that the determinant of the matrix must be equal to zero and the trace of the matrix must also be equal to zero.

Can a nilpotent matrix have non-zero entries?

Yes, a nilpotent matrix can have non-zero entries. However, the entries must follow a specific pattern in order for the matrix to be nilpotent.

Is the zero matrix considered a nilpotent matrix?

Yes, the zero matrix is considered a nilpotent matrix since any power of the zero matrix will still result in a zero matrix.

What is the significance of nilpotent matrices in mathematics?

Nilpotent matrices are important in linear algebra as they help in understanding the properties of matrices and their behavior under multiplication. They are also used in various applications such as in solving differential equations and in the study of group theory.

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