Hi,
I have used "eig" function to calculate the eigenvalues of a 6×6 symmetric definite positive matrix and this is the result:
-1,1574e-05
-4,8340e-06
2,0876e-05
6,1373e+10
7,76111e+10
16,3586e+10
Why some values are negative? I know they can be considered zero respect others but for this type of matrix all eigenvalues should be positive! What is the "problem"?
I have also used "eig" function to calculate eigenvalues and eigenvectors for a FEM 2D vibration problem (32 degree of freedom) and the first mode of vibration is out of phase (only some elements). The deformed shape is ok but this mode should be in phase. Can this result be influenced by numerical Matlab computation of corresponding eigenvalue?
Thanks for your help Mark
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