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

[Jose E. Roman]

El 15/04/2013, a las 16:50, Zhang Wei escribió:

> Hi 
> Since the case is not that big(size of matrix is 9600x9600. ). I have done test in the first way, which gives me a 2.1 Gb ASCII file. The eigen values I  get from matlab are :
>> In eigs>processEUPDinfo at 1340
>  In eigs at 357 
>>> c
> 
> c =
> 
>        0          
>        0          
>        0          
>        0          
>   1.9405 - 0.4733i
>   1.9405 + 0.4733i
> 
> And what I got from slepc are :
> 
> 0       1.99988
> 1       1.99974 
> 2       1.99971 
> 3       1.99913+0.0370552j      
> 4       1.99913+-0.0370552j     
> 5       1.99894+0.0370115j      
> 
> Non of them is right one. And on the other hand there are lots of small values in the matrix which are almost in order of 1e-7 and even smaller.  
> Could it  be the reason resulting such problem?


Note that if you are trasferring the matrix via a text file, then the matrix loaded in Matlab will differ slightly form the one being handled by SLEPc, so you can expect differences in eigenvalues. For a more equivalent computation you should transfer the matrix in binary format.

Anyway, my impression is that you have a bug in the code that generates the matrix, so I would suggest checking that. Having small nonzero entries in the matrix should not be a problem for the eigensolver.

Jose