3×3 matrix B has eigenvalues 0, 1 and 2. Find the rank of B.
I understand that 0 being an eigenvalue implies that rank of B is less than 3.
The solution is here (right at the top). It says that rank of B is 2 because the number of distinct non zero eigenvalues is 2.
This thread says that the only information that the rank gives is the about the eigenvalue 0 and its corresponding eigenvectors.
What am I missing?
Edit
What I am really looking for is an explicit answer to this:
"Is the rank of a matrix equal to the number of distinct non zero eigenvalues of that matrix?"
Yes/No/Yes for some special cases
Answer
Take for example the matrix A=[100020000], its rank is obviously 2, eigenvalues are distinct and are 0,1,2.
We have theorem which says that if eigenvalues are distinct then their eigenvectors must be linearly independent, but the rank of the matrix is n−1 if one of this eigenvalues is zero.
Edit after question edit
To answer more generally we need Jordan forms.
Let A be n-dimensional square matrix with n−k non-zero eigenvalues
(I don't make here a distinction between all different values and repeated- if all are distinct then there are n−k values , if some are with multiplicities then summarize their number with multiplicities to make full sum n−k) and k zero eigenvalues.
Express A through similarity with the Jordan normal form A=PJP−1.
The matrix J can be presented as composition J=[Jn00Jz] where Jn is a square part of Jordan matrix with n−k non-zero values (which are eigenvalues) on diagonal and Jz is a square part of Jordan matrix with k zero values on its diagonal.
It is clear that because on the diagonal of upper-triangular matrix Jn are non-zero values and the determinant as the product of these values is non-zero so Jn has full rank i.e. n−k.
The rank of Jz depends on the detailed form of this matrix and it can be from 0 (when Jz=0) to k−1 ( see examples in comments). It can't be k because Jz is singular.
Therefore the final rank of J can be from n−k to n−1.
Similarity preserves rank so the rank of A can be also from n−k to n−1.
No comments:
Post a Comment