# BDA3 Chapter 3 Exercise 13

Here’s my solution to exercise 13, chapter 3, of Gelman’s Bayesian Data Analysis (BDA), 3rd edition. There are solutions to some of the exercises on the book’s webpage.

Suppose we have likelihood $$y_i \mid \mu \sim \dnorm(\mu, \Sigma)$$ and prior $$\mu \sim \dnorm(\mu_0, \Lambda_0)$$, where the distributions are multivariate in each case. This is the multivariate analogue of question 9, where we found that the quadratic form solution was simpler. The quadratic forms of the log-likelihoods and log-prior are

\begin{align} Q_i(\mu) &= (y_i - \mu)^T \Sigma^{-1} (y_i - \mu) \\ Q_0(\mu) &= (\mu_0 - \mu)^T \Lambda_0^{-1} (\mu_0 - \mu) . \end{align}

The quadratic form of the full log-likelihood is thus $$Q_1^n := \sum_{i = 1}^n Q_i$$, given by

\begin{align} Q_1^n(\mu) &= (a_1^n - \mu)^T \left( n \Sigma^{-1} \right) (a_1^n - \mu) + c_1^n \\ a_1^n &= \left( n\Sigma^{-1} \right)^{-1} \sum_{i = 1}^n \Sigma^{-1} y_i \\ &= \left( n\Sigma^{-1} \right)^{-1} \left( n\Sigma^{-1} \bar y \right) , \end{align}

where $$c_1^n$$ is constant and can thus be ignored. The expression for $$a_1^n$$ can easily be proved by induction. Indeed, for $$n = 1$$ we have $$a_1^1 = y_1$$. In the induction step, we can assume $$a_1^{n - 1} = \left( (n - 1)\Sigma^{-1} \right)^{-1} \left( \Sigma^{-1} \sum_1^{n-1} y_i \right)$$. Then we have

\begin{align} a_1^n &= \left( (n - 1)\Sigma^{-1} + \Sigma^{-1} \right)^{-1} \left( \Sigma^{-1} \sum_{i = 1}^{n-1} y_i + \Sigma^{-1} y_n \right) \\ &= \left( n\Sigma^{-1} \right)^{-1} \left( \Sigma^{-1} \sum_{i = 1}^{n} y_i \right) \\ &= \left( n\Sigma^{-1} \right)^{-1} \left( n\Sigma^{-1} \bar y \right) . \end{align}

The quadratic form of the log-posterior is then $$Q = Q_0 + Q_1^n$$, given by

\begin{align} Q(\mu) &= (\mu_n - \mu)^T \Lambda_n^{-1} (\mu_n - \mu) + c \\ \Lambda_n^{-1} &= \lambda_0^{-1} + n\Sigma^{-1} \\ \mu_n &= \left( \Lambda_0^{-1} + n\Sigma^{-1} \right) \left( \lambda_0^{-1}\mu_0 + n\Sigma^{-1} \bar y \right) , \end{align}

where $$c$$ is constant and can be dropped.