Created attachment 573 [details]
Test code to demonstrate the bug
Using the SelfadjointEigenSolver for a multiple of the 2x2 fixed-size identity matrix gives correct eigenvalues, but eigenvectors full of 'nan'.
In the routine in 'src/Eigenvalues/SelfAdjointEigenSolver.h' there are specialised routines for 3x3 and 2x2 matrices. In the 3x3 case there is a check for equal eigenvalues
// really compute the eigenvectors
, but in the routine for 2x2 this check is missing. It follows a division by zero in the normalisation of the vectors.
Created attachment 574 [details]
thank you for the report, the fix is slightly more subtle, I'll push a fix soon.