On Feb 19, 2013, at 12:53 PM, Marcelo Xavier Guterres <m.guterres at gmail.com> 
wrote:

> Esteemed colleague,
> 
> thanks for the previous answer.  But still can not understand one aspect.
> 
> 
> The GMRES behaves well up to a point, but then the quality of the final 
> solution is bad, compared to analytical solution.
> 
> 
> It is a problem in the method or truncation error?

No, it is an approximate solver and it gets worse as the problem size 
increases.  CG gets worse also.  CG is just better for this problem.

> 
> 
> m     n       m x n   error ( KSPCG x PCJACOBI )      error ( KSPGMRS x 
> PCJACOBI )
> 3     3       9       1,92E?16        4,42E?16        
> 
> 4     4       16      2,08E?16        6,46E?16        
> 
> 5     5       25      4,41E?16        9,63E?16        
> 
> 6     6       36      8,77E?16        8,26E?16        
> 
> 7     7       49      2,37E?06        2,52E?06        
> 
> 8     8       64      1,17E?05        1,33E?05        
> 
> 9     9       81      9,32E?06        1,26E?05        
> 
> 10    10      100     7,33E?06        9,93E?06        
> 
> 20    20      400     4,22E?05        3,16E?04        
> 
> 30    30      900     1,06E?04        2,37E?02        
> 
> 40    40      1600    2,14E?04        3,77E?02        
> 
> 50    50      2500    3,37E?04        9,36E?03        
> 
> 100   100     10000   1,27E?03        6,53E?02        
> 
> 200   200     40000   4,32E?03        3,67E?01        
> 
> 300   300     90000   9,53E?02        1,02E+00        
> 
> 400   400     160000  1,68E?02        2,10E+00        
> 
> 500   500     250000  2,61E?02        3,66E+00        
> 
> 
> 
> 
> -- 
> Sauda??es Pampeanas
> -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
> 
> Phd. student Marcelo

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