[petsc-users] qep results different from polyeig in matlab

Tue Sep 3 23:42:43 CDT 2013

```Hello
every one. I am Wei. I want to use slepc to solver my quadratic eigenvalue problem. and now I start with the tutorial ex16 and compare with polyeig function matlab. but the two give different results.
here is the results from slepc

Number of requested eigenvalues:  6
Stopping condition: tol= 1e-08 , maxit= 100
Number of converged approximate eigenpairs:  8

k           ||(k^2M+Ck+K)x||/||kx||
-------------------- -------------------------
-7.85046229415e-16 + 2.81636155327 J, 3.31272023702e-09
-7.85046229415e-16 + -2.81636155327 J, 3.31272023702e-09
6.28036983474e-16 + 2.79837285903 J, 2.31471998545e-09
6.28036983474e-16 + -2.79837285903 J, 2.31471998545e-09
-5.4953236053e-16 + 2.78026777804 J, 6.1734628482e-10
-5.4953236053e-16 + -2.78026777804 J, 6.1734628482e-10
7.85046229433e-17 + 2.76882293959 J, 8.38075971231e-10
7.85046229433e-17 + -2.76882293959 J, 8.38075971231e-10

and here is the results from polyeig in Matlab.
tol =

1.0000e-008

ee =

0 + 2.4425i
0 - 2.4425i
0 + 2.4218i
0 - 2.4218i
0 + 2.3876i
0 - 2.3876i
0 + 2.3405i
0 - 2.3405i
0 + 2.2815i
0 - 2.2815i
0 + 2.2117i
0 - 2.2117i
0 + 2.1324i
0 - 2.1324i
0 + 2.0456i
0 - 2.0456i
0 + 1.4262i
0 - 1.4262i
0 + 1.4612i
0 - 1.4612i
0 + 1.5164i
0 - 1.5164i
0 + 1.5881i
0 - 1.5881i
0 + 1.9533i
0 - 1.9533i
0 + 1.6717i
0 - 1.6717i
0 + 1.7631i
0 - 1.7631i
0 + 1.8581i
0 - 1.8581i

it is different a lot especially for imagine part even if both converged to 1e-8. I tried both linear solver and qarnoldi method.   could any one tell me what is that? is it error?

---------------------------------------

Wei zhang

PhD

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