I have a very large symmetric matrix (50000 x 50000). I'm wondering how I can store this matrix efficiently. Is there any matrix type something like SymmMatrix which only store the upper or lower triangular part of the matrix?

Thanks.

If your matrix is sparse the sparse linear algebra routines may be useful; see http://eigen.tuxfamily.org/dox/group__S ... apter.html .

If your matrix is not sparse, you can (as you say) save half the memory, but this is not yet supported in Eigen; see the entry http://eigen.tuxfamily.org/bz/show_bug.cgi?id=42 in bugzilla. Contributions to get this working in Eigen would be gratefully accepted

My matrix is not sparse. Thanks for letting me know that.