New user self-registration is currently disabled. Please email eigen-core-team @ if you need an account.
Bug 1013 - Eigenvalues 2x2 matrix
Eigenvalues 2x2 matrix
Product: Eigen
Classification: Unclassified
Component: Eigenvalues
x86 - 64-bit Linux
: Normal Wrong Result
Assigned To: Nobody
Depends on:
  Show dependency treegraph
Reported: 2015-05-07 10:19 UTC by thomas.rueberg
Modified: 2015-05-07 14:33 UTC (History)
4 users (show)

Test code to demonstrate the bug (732 bytes, text/x-c++src)
2015-05-07 10:19 UTC, thomas.rueberg
no flags Details
Patch (712 bytes, patch)
2015-05-07 10:25 UTC, thomas.rueberg
no flags Details | Diff

Description thomas.rueberg 2015-05-07 10:19:51 UTC
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

else {
  // 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.
Comment 1 thomas.rueberg 2015-05-07 10:25:48 UTC
Created attachment 574 [details]
Comment 2 Gael Guennebaud 2015-05-07 13:41:04 UTC
thank you for the report, the fix is slightly more subtle, I'll push a fix soon.

Note You need to log in before you can comment on or make changes to this bug.