# Find joint distribution of same-mean normal variables

Suppose I have a variable $$C$$ and a set of $$N$$ variables $$L_1, L_2, ..., L_N$$ that are distributed according to:

\begin{aligned} C &\sim \mathcal N \left(\mu, \delta^2 \right) \\ \forall i. L_i | C=c &\sim \mathcal N\left(c, \sigma_i^2 \right) \end{aligned}

That is, conditional on knowing the value of $$C$$, the $$L_i$$ are normally independently distributed with mean $$c$$ and variance $$\sigma_i^2$$. This implies that the vector $$\vec L = (L_1, ..., L_N)^T$$ is distributed according to a N-dimensional multivariate normal with mean vector $$\vec \mu$$ and covariance matrix $$\mathbf \Sigma$$.

When $$N=1$$, it is straightforward to see that $$L_1 \sim \mathcal N \left(\mu, \delta^2 + \sigma_1^2 \right)$$. It seems to me like this should imply that $$\vec \mu = (\mu, ..., \mu)^T$$, and that the diagonal elements of $$\mathbf \Sigma$$ should be $$\delta^2 + \sigma_i^2$$. The off-diagonal elements of the covariance matrix are, of course, not zero, since the variables $$L_i$$ are not independent when I'm not conditioning on $$C$$.

Is this true, though? Is there a way for me to prove this in a less tedious way than algebraically working everything out? What are the values of the off-diagonal elements?

• "are normally i.i.d. with mean c" is not quite accurate, since they are (conditionally) independent, but not identical because the $\sigma_i$ may differ. You might want to edit this detail. – iljusch Aug 8 '19 at 7:48
• You're right, I've fixed that, thanks! – Pedro Carvalho Aug 8 '19 at 14:32

You can think of the $$L_i$$ as being sums of the form $$L_i = C + X_i$$, where $$X_i$$ are independent random variables with $$X_i\sim\mathcal N(0,\sigma_i^2)$$ that are also independent of $$C$$. Therefore, for $$\mathcal L = (L_1,\dots,L_N)^T$$, $$\mathcal C = (C,\dots,C)^T$$ and $$\mathcal X = (X_1,\dots,X_N)^T$$, you have $$\mathcal L = \mathcal C + \mathcal X$$. Note that $$\mathcal X\sim \mathcal N\left( \begin{pmatrix} 0\\ \vdots\\ 0 \end{pmatrix} , \begin{pmatrix} \sigma_1^2 & & 0 \\ &\ddots& \\ 0& & \sigma_N^2 \end{pmatrix} \right), \qquad \mathcal C\sim \mathcal N\left( \begin{pmatrix} \mu\\ \vdots\\ \mu \end{pmatrix} , \delta^2 \begin{pmatrix} 1 &\cdots & 1 \\ \vdots&\ddots&\vdots \\ 1& \cdots & 1 \end{pmatrix} \right)$$ Therefore, $$\mathcal L\sim \mathcal N\left( \begin{pmatrix} \mu\\ \vdots\\ \mu \end{pmatrix} , \begin{pmatrix} \delta^2+\sigma_1^2 &\cdots & \delta^2\\ \vdots&\ddots&\vdots \\ \delta^2& \cdots & \delta^2+\sigma_N^2 \end{pmatrix} \right)$$ So, your reasoning is correct and the off-diagonal elements are all equal to $$\delta^2$$.
• Thank you! Also, minor nitpick, should it be $\mathcal X = (X_i, ..., X_N)^T$? – Pedro Carvalho Aug 8 '19 at 14:36