[eigen] Mapping a dynamic vector - howto

[Helmut Jarausch <jarausch@xxxxxxxxxxxxxxxxxxx>]

Hi,

in several contexts (e.g. PDE) one has a (long)

VectorXd X(N)

where one knows that  N = m*m .  ( m, N unknown at compile time)

Often one wishes to interpret this vector as an m x m matrix (for  
reading and writing).
How can this be achieved with Eigen?

Many thanks for a hint,
Helmut.