lapack routine dsygv giving negative eigen values

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lapack routine dsygv giving negative eigen values

Postby rajithkr » Sun Mar 19, 2017 5:18 am

Hello,
just joined this forum as i got into a problem while using LAPACK routine (DSYGV). i am using fortran 95 for an eigen value analysis for type Ax=lamda B type problem. i am trying to get the eigen value using the routine, and it gives first two eigen values as -ve which is not expected. i ran previously on 5 similar models and i got the eigen values as expected.. what could be reason for this negative value in eigen value listing. I checked both matrices - A and B, they are square, symmetric and all the diagonal elements are positive elements in all 6 cases. i could upload the matrices if anyone wish to see them - they are 660 X660 matrices - around 11mb each in csv format. appreciate any guidance here.
rajithkr
 
Posts: 2
Joined: Sun Mar 19, 2017 3:48 am

Re: lapack routine dsygv giving negative eigen values

Postby rajithkr » Tue Mar 21, 2017 2:07 am

Hello,
just joined this forum as i got into a problem while using LAPACK routine (DSYGV). i am using fortran 95 for an eigen value analysis for type Ax=lamda B type problem. i am trying to get the eigen value using the routine, and it gives first two eigen values as -ve which is not expected. i ran previously on 5 similar models and i got the eigen values as expected.. what could be reason for this negative value in eigen value listing. I checked both matrices - A and B, they are square, symmetric and all the diagonal elements are positive elements in all 6 cases. i could upload the matrices if anyone wish to see them - they are 660 X660 matrices - around 11mb each in csv format. appreciate any guidance here. I took out the matrices A and B into Mathcad and checked for cholesky decomposition and Eigen values, they work well and gave Eigen values as what is expected. upon comparing the Eigen listing in Mathcad and dsygv listing, I found the following.
eigen value(3) in dsygv was matching eigenval(1) of mathcad, eigenval(4) was matching eigenval(2) of mathcad and so on.. looks like the 2 negative values got inserted in the listing - attached is the screenshot. could some one explain why this is happening
comparison.JPG
comparison.JPG (163.11 KiB) Viewed 5 times
rajithkr
 
Posts: 2
Joined: Sun Mar 19, 2017 3:48 am


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