$A$ is a matrix, $C$ is a positive definite matrix (denoted by $C>0$), and $K=A^T C A$. We need to prove two parts: (a) $\operatorname{ker} K=\operatorname{coker} K=\operatorname{ker} A$ and (b) $\operatorname{img} K=\operatorname{coimg} K=\operatorname{coimg} A$.
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