I am a masters student in controls and would like to get insight into the concept of relative gain array for multivariable feedback control. In general what I have come across from the book on the same topic by Skogestad and Postlethwaite, relative gain array element $\lambda_{i,j}$ is given by $G_{i,j}G^{-1}_{j,i}$, where G is a square matrix.
But how to find it if the matrix G is not invertible? In this case the matrix is $G = \left[\begin{matrix} 1 & 1 \\ 1 & 1\end{matrix}\right]$. Is there a different approach I can take, a different definition perhaps to calculate the RGA?
The concept of Moore-Pensrose pseudoinverse can be used to approximate for some properties of inverse in this case.