Eigen
3.3.7

This module is currently for internal use only
It defines various builtin and external ordering methods for sparse matrices. They are typically used to reduce the number of elements during the sparse matrix decomposition (LLT, LU, QR). Precisely, in a preprocessing step, a permutation matrix P is computed using those ordering methods and applied to the columns of the matrix. Using for instance the sparse Cholesky decomposition, it is expected that the nonzeros elements in LLT(A*P) will be much smaller than that in LLT(A).
Usage :
A simple usage is as a template parameter in the sparse decomposition classes :
It is possible as well to call directly a particular ordering method for your own purpose,
Classes  
class  Eigen::AMDOrdering< StorageIndex > 
class  Eigen::COLAMDOrdering< StorageIndex > 
class  Eigen::NaturalOrdering< StorageIndex > 