Fun with determinants

Petersen and Pedersen (2012) note the standard identities:

Let \(\mathbf{A}\) be an \(n \times n\) matrix. \[ \begin{aligned} \operatorname{det}(\mathbf{A}) &=\prod_{i} \lambda_{i} \quad \lambda_{i}=\operatorname{eig}(\mathbf{A}) \\ \operatorname{det}(c \mathbf{A}) &=c^{n} \operatorname{det}(\mathbf{A}), \quad \text { if } \mathbf{A} \in \mathbb{R}^{n \times n} \\ \operatorname{det}\left(\mathbf{A}^{T}\right) &=\operatorname{det}(\mathbf{A}) \\ \operatorname{det}(\mathbf{A} \mathbf{B}) &=\operatorname{det}(\mathbf{A}) \operatorname{det}(\mathbf{B}) \\ \operatorname{det}\left(\mathbf{A}^{-1}\right) &=1 / \operatorname{det}(\mathbf{A}) \\ \operatorname{det}\left(\mathbf{A}^{n}\right) &=\operatorname{det}(\mathbf{A})^{n} \\ \operatorname{det}\left(\mathbf{I}+\mathbf{u v}^{T}\right) &=1+\mathbf{u}^{T} \mathbf{v} \end{aligned} \] For \(n=2\) : \[ \operatorname{det}(\mathbf{I}+\mathbf{A})=1+\operatorname{det}(\mathbf{A})+\operatorname{Tr}(\mathbf{A}) \] For \(n=3\) : \[ \operatorname{det}(\mathbf{I}+\mathbf{A})=1+\operatorname{det}(\mathbf{A})+\operatorname{Tr}(\mathbf{A})+\frac{1}{2} \operatorname{Tr}(\mathbf{A})^{2}-\frac{1}{2} \operatorname{Tr}\left(\mathbf{A}^{2}\right) \] For \(n=4\) : \[ \begin{aligned} \operatorname{det}(\mathbf{I}+\mathbf{A})=& 1+\operatorname{det}(\mathbf{A})+\operatorname{Tr}(\mathbf{A})+\frac{1}{2} \\ &+\operatorname{Tr}(\mathbf{A})^{2}-\frac{1}{2} \operatorname{Tr}\left(\mathbf{A}^{2}\right) \\ &+\frac{1}{6} \operatorname{Tr}(\mathbf{A})^{3}-\frac{1}{2} \operatorname{Tr}(\mathbf{A}) \operatorname{Tr}\left(\mathbf{A}^{2}\right)+\frac{1}{3} \operatorname{Tr}\left(\mathbf{A}^{3}\right) \end{aligned} \] For small \(\varepsilon\), the following approximation holds: \[ \operatorname{det}(\mathbf{I}+\varepsilon \mathbf{A}) \cong 1+\operatorname{det}(\mathbf{A})+\varepsilon \operatorname{Tr}(\mathbf{A})+\frac{1}{2} \varepsilon^{2} \operatorname{Tr}(\mathbf{A})^{2}-\frac{1}{2} \varepsilon^{2} \operatorname{Tr}\left(\mathbf{A}^{2}\right) \]

For a block matrix we have: For \(n=4\) : \[ \begin{aligned} \operatorname{det}(\mathbf{I}+\mathbf{A})=& 1+\operatorname{det}(\mathbf{A})+\operatorname{Tr}(\mathbf{A})+\frac{1}{2} \\ &+\operatorname{Tr}(\mathbf{A})^{2}-\frac{1}{2} \operatorname{Tr}\left(\mathbf{A}^{2}\right) \\ &+\frac{1}{6} \operatorname{Tr}(\mathbf{A})^{3}-\frac{1}{2} \operatorname{Tr}(\mathbf{A}) \operatorname{Tr}\left(\mathbf{A}^{2}\right)+\frac{1}{3} \operatorname{Tr}\left(\mathbf{A}^{3}\right) \end{aligned} \] For small \(\varepsilon\), the following approximation holds: \[ \operatorname{det}(\mathbf{I}+\varepsilon \mathbf{A}) \cong 1+\operatorname{det}(\mathbf{A})+\varepsilon \operatorname{Tr}(\mathbf{A})+\frac{1}{2} \varepsilon^{2} \operatorname{Tr}(\mathbf{A})^{2}-\frac{1}{2} \varepsilon^{2} \operatorname{Tr}\left(\mathbf{A}^{2}\right) \]