Help with SSYEV

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Help with SSYEV

Postby vibhav » Fri Sep 08, 2006 10:09 pm

Hi All,

I am new to this forum and fortran. I am having a simple problem using lapack.

I am trying to solve for eigen-vector and value for a real symmetric matrix (positive definite). I am using SSYEV/DSYEV function.

For my matrix I am suppose to get all the positive eigen-values. But I some of my eigenvalues are negative.

Can anyone help me out with this or point me in the right direction. Is there other functions which is better than SSYEV?


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Postby Julien Langou » Sun Sep 10, 2006 8:35 pm


how much "negative" are your eigenvalues?

If the absolute value of your negative eigenvalue is smaller than machine epsilon times the norm of A, you should consider this eigenvalue to be zero. (Which make your matrix numerically not definite so you still might not be happy ...)

If you are looking for more accurate symmetric eigensolvers, you should give a try to xSYEVX, but this will not help that much. The main problem is that with the current LAPACK solvers you loose componentwise accuracy when we go to tridiagonal form. You are normwise accurate. Some new solvers are developped right now to remedy this problem.

Julien Langou
Julien Langou
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