I'm working on a homework problem, and I'm stuck. I guess my linear algebra still needs some work...
I've arrived at
$\mathbf{D}= \left[ \begin{matrix} \mathbf{C} & \mathbf{1}^T \\ \mathbf{1} & 0 \end{matrix} \right] $
where $\mathbf{C}$ is a $n$ by $n$ matrix and $\mathbf{1}$ is a $n$ by $1$ vector of all ones. I need to find $\mathbf{D}^{-1}$. Can I express it in terms of $\mathbf{C}^{-1}$? Can I proceed at all? Does it help if $\mathbf{C}$ is symmetric?
Thanks