Drexel dragonThe Math ForumDonate to the Math Forum



Search All of the Math Forum:

Views expressed in these public forums are not endorsed by Drexel University or The Math Forum.


Math Forum » Discussions » Software » comp.soft-sys.matlab

Topic: Problem computing determinant
Replies: 1   Last Post: Aug 18, 1996 10:40 PM

Advanced Search

Back to Topic List Back to Topic List Jump to Tree View Jump to Tree View   Messages: [ Previous | Next ]
Craig C. Tierney

Posts: 1
Registered: 12/11/04
Problem computing determinant
Posted: Aug 16, 1996 5:35 PM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply

I am having a problem computing the determinant of a matrix.
Specifically, I am solving an eigenvalue problem, and then using
the original matrix and one of the eigenvalues and checking
that the determinant is zero.

n=5;
A=random('T',1,n,n);
[x,l]=eig(R);
det(A-eye(size(A))*l(1,1))

From det(A-lI)=0, the eigenvalues, l, can be found. So The result
of the last line of the script should be close to zero. This works
acceptable for small matricies (n<8). When n>10, the value of the
determinant is on the order of 0.1-1. Here the result is dependent
on the numbers generated by the random statement.

For n=100, I check the orthogonality of the eigenvectors, and the
results from the diagonalization of the system.
For orthogonality and diagonalization, the results are accurate to at
least 10e-6. However the determinant of A-lI for any of the eigenvalues
found is ~10e260!

Am I trying to do something real stupid, or is it really roundoff error.
Does the determinant function return the product of the diagonal elements
of U from an LU decomposition?

It is hard to believe that the eigenvectors are that accurate, but the
determinant cannot be found as accurate.

Thanks,
Craig





Point your RSS reader here for a feed of the latest messages in this topic.

[Privacy Policy] [Terms of Use]

© Drexel University 1994-2014. All Rights Reserved.
The Math Forum is a research and educational enterprise of the Drexel University School of Education.