### zgeev() eigenvalue ordering in Lapack

Posted:

**Wed Apr 27, 2005 11:42 am**Hi

I'm using zgeev() from LAPACK to solve an eigenvalue/vector problem. However the documentation is not clear on the ordering of the eigenvalues the function outputs. My problem is that I'm solving the system as a function of two parameters, called say 's' and 'r', and I want to look at how the derivatives of one of the eigenvalues changes as a function of 's' and 'r'. this eigenvalue is the one which is zero (null) for s=r= 1 (the details arn't important).

previously I just looked at the eigenvalues for s=r=1, found which one was zero, and looked at the same position in the vector of eigenvalues for other values of 's' and 'r', assuming this corresponded to the same eigenvalue at the corresponding different values of s and r.

However now I have made some changes to my matrix and it appears that the eigenvalue I am trying to keep track of is no longer held in the same position in the vector as I change s and r.... or at least this is my guess as to why my derivatives don't work any more!

I hope this question is clear...

thanks,

Neill

I'm using zgeev() from LAPACK to solve an eigenvalue/vector problem. However the documentation is not clear on the ordering of the eigenvalues the function outputs. My problem is that I'm solving the system as a function of two parameters, called say 's' and 'r', and I want to look at how the derivatives of one of the eigenvalues changes as a function of 's' and 'r'. this eigenvalue is the one which is zero (null) for s=r= 1 (the details arn't important).

previously I just looked at the eigenvalues for s=r=1, found which one was zero, and looked at the same position in the vector of eigenvalues for other values of 's' and 'r', assuming this corresponded to the same eigenvalue at the corresponding different values of s and r.

However now I have made some changes to my matrix and it appears that the eigenvalue I am trying to keep track of is no longer held in the same position in the vector as I change s and r.... or at least this is my guess as to why my derivatives don't work any more!

I hope this question is clear...

thanks,

Neill