Adam Wheeler

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Math and physics notes

matrix identities

$(ABC\dots)^{-1} = C^{-1} B^{-1} A^{-1}$.
$(ABC\dots)^T = C^T B^T A^T$.
$\vert c A \vert = c^n \vert A\vert$ where A is $ n \times n $.

linear regression

The Moorse-Penrose pseudoinverse for orthogonal real matrices is

$ A^+ = (A^T A)^{-1} A^T $

The normal equation is

$\widehat\beta = (X^T \Sigma^{-1} X)^{-1} X^T \Sigma^{-1} y$

The hat matrix is given by

$ X (X^T \Sigma^{-1} X)^{-1} X^T \Sigma^{-1} $

matrix decompositions

Singular Value decomposition: $A = U D V^T$ where

The columns of $U$ are the eigenvectors of $A A^T$
$D$ is a diagonal matrix containting hte eigenvalues of $A A^T$ (the singular values)
$V$ is the matrix whose columns are the eigenvectors of $A^T A$

LU decomposition: $A = LU$ where

$L$ is lower-triangular
$U$ is upper-triangular

The Normal distribution

$$\mathcal{N}(\mathbf{x} | \mathbf{\mu}, \Sigma) = \exp\left( - \frac{1}{2} \mathbf{r}^T \Sigma^{-1} \mathbf{r} \right) , |2 \pi \Sigma|^{-\frac{1}{2}}$$

where $\mathbf{r} = \mathbf{x} - \mathbf{\mu}$. Note that the $2 \pi$ is inside the determinant.

linear operations on Gaussian random variables: If $ x \sim \mathcal{N}(\mu_x, \Sigma_X)$, and $y \sim \mathcal{N}(\mu_y, \Sigma_y)$, then

$x + y \sim \mathcal{N}(\mu_x + \mu_y, \Sigma_x + \Sigma_y) $.
$Ax \sim \mathcal{N}(A \mu_x, A \Sigma_x A^T)$.

Product of Gaussian PDFs: $\mathcal{N}(\mathbf{x} \vert \alpha, \Sigma) \mathcal{N}(\mathbf{x} \vert \beta, \Omega) = \eta \mathcal{N}(\mathbf{x} | \mathbf{m}, C)$, where

$ \mathbf{m} = (\Sigma^{-1} + \Omega^{-1})^{-1} (\Sigma^{-1} \alpha + \Omega^{-1} \beta) $.
$ C = (\Sigma^{-1} + \Omega^{-1})^{-1}$.
$ \eta = \mathcal{N}(\alpha-\beta \vert 0, \Sigma + \Omega) $.

Refactoring the product of heirarchical Gaussian PDFs: $\mathcal{N}(\mathbf{x} \vert M \theta, C) \mathcal{N}(\theta \vert \mu, \Lambda) = \mathcal{N}(\theta \vert \mathbf{a}, A) \mathcal{N}(\mathrm{x} \vert \mathrm{b}, B)$, where

$A^{-1} = \Lambda^{-1} + M^T C^{-1} M$.
$ a = A(\Lambda^{-1} \mu + M^T C^{-1} \mathbf{x})$.
$B = C + M^T \Lambda M$.
$b = M \mu$.

Sources: The Matrix Cookbook, Hogg+ 2020