I have the following equation: $$GCV(\lambda)=\frac{ n\times\text{trace}\{Y'[I-S_{\phi,\lambda}]^{-2}Y\} }{\{\text{trace}[I-S_{\phi,\lambda}]\}^2}$$
I know $I-S_{\phi,\lambda}$ is a matrix what is the power to the -2 supposed to mean? is it the power of each element?