I'm reading the paper "Numerical solution of saddle point problems" by Michele Benzi. In the abstract, he states that these types of large linear systems of saddle point are challenging due to their indefiniteness and often poor spectral properties.
And I cannot understand what does he means with indefiniteness (what makes this type of linear system have this characteristic? what this implies to the system?)and with poor spectral properties (what makes this type of linear system have this characteristic? what this implies to the system?)
I'm new with the subject of matter, if is self-evident please be kind.
Thank you for your help.