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Let us consider the $2n \times 2n$ block matrix $\mathbf{X} = \pmatrix{\mathbf{P} & \mathbf{Q}\\ \mathbf{R} & \mathbf{S} }$ where $\mathbf{P} , \mathbf{Q} , \mathbf{R} , \mathbf{S}$ are square matrices of order $n$ which commute pairwise. Show that $\det \mathbf{X} = \det (\mathbf{PS} - \mathbf{QR}).$

The problem in this case I am having is when none of the matrices $ \mathbf{P} , \mathbf{Q} , \mathbf{R} , \mathbf{S} $ are invertible. Any help will be greatly appreciated.

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    Maybe this helps: [Determinant of Bloc$k$ matrices](http://en.wikipedia.org/wiki/Determinant#Block_matrices) and the linked pdf with a proof: [blocks.pdf](http://www.mth.kcl.ac.uk/~jrs/gazette/blocks.pdf)2012-08-31

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It's enough for $P$ and $R$ to commute. When $P$ is invertible,

$\det(X) = \det(PS - PRP^{-1}Q)$. If $P$ and $R$ commute and $P$ is not invertible, consider

$f(t) = \det \pmatrix{P+tI & Q\cr R & S\cr}$

For all but finitely many $t$, $P+tI$ is invertible, and it commutes with $R$, so $f(t) = \det((P+tI)S - (P+tI) R (P+tI)^{-1} Q) = \det((P+tI)S - RQ)$ Now both $f(t)$ and $\det((P+tI)S - RQ)$ are polynomials in $t$, so if they are equal for all but finitely many $t$ they are equal for all $t$.

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    How many eigenvalues can $P$ have?2012-08-31