[petsc-users] algorithm to get pure real eigen valule for general eigenvalue problem(Non-hermitian type)

[Zhang Wei]

Hi!
Thanks for your reply. I am now dealing with non symmetric  eigen problem, where I expect to get these complex pairs. I am looking for the largest magnitude eigen pairs. But I always get many pure real eigen pairs,which in my case make no sense. The thing is that these are not totally wrong.since it is a standard case I compared with others, these real eigen values are somehow close to the magnitude of "correct" results.for eigen vectors, the "correct" results can be express as "T*exp(kx)",where the T is chebyshev polynomial. And all real eigen vectors I got are extract chebyshev polynomial. Actually I already set the problem to NHEP,and can get some complex eigen pairs. Comparing with expected one those eigen values are larger in angle. On the other hand I set the tolerance to 1e-9.

Thanks in advance!

Yours Sincerely
------------------------
Wei Zhang
Ph.D
Hydrodynamic Group
Dept. of Shipping and Marine Technology
Chalmers University of Technology
Sweden
Phone:+46-31 772 2703

On 14 apr 2013, at 16:40, "Jose E. Roman" <jroman at dsic.upv.es> wrote:

> 
> El 12/04/2013, a las 15:22, Zhang Wei escribió:
> 
>> Dear Sir:
>> I am using slepc for hydrodynamic instability analysis. From my understanding most of these unstable modes in hydrodynamic are large scale structure with low frequency, which means very small angle for the complex eigenvalue. Actually I tried a channel flow case, finally I got several Eigen value with imaginary part is exact 0. and from the Eigen modes,  it gives "correct" results in one direction but not wave, since the wave number is 0. So I suppose there must be algorithm to control so. Does any one know what could influence so?
>> 
>> 
>> Best Regards
>> 
>> Wei                   
> 
> I don't understand what your problem is. Can you express it in terms of the algebraic eigenproblem? Are you getting wrong solutions for the eigenproblem? Did you check the residual of the eigenpairs?
> 
> Jose
>