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Let $\mathbf{X} = (X_1, \dots, X_p)^\mathsf{T}$ be a $p$-dimensional random variable following a multivariate normal distribution with mean vector $\boldsymbol{\mu}$ and covariance matrix $\boldsymbol{\Sigma}$, i.e. $\mathbf{X} \sim \mathcal{N}(\boldsymbol{\mu}, \boldsymbol{\Sigma})$.

Let $X_{(1)}$ the minimum of the components of $\mathbf{X}$, that is to say, $X_{(1)} = \min\{X_1,\dots,X_p\}$.

What are the cumulative distribution function (cdf) and the probability distribution function (pdf) of $X_{(1)}$?

I've read about the distribution of the maximum here and also here. I guess that both concepts must be tightly connected, since $\min\{x_1,\dots,x_p\} = - \max\{-x_1,\dots,-x_p\}$.

However, I still have doubts. I read this expression somewhere, but I think it is not totally right, or at least I am not able to deduce it by my own:

The cdf of $X_{(1)}$ is

$F(x) = G\left( x \mathbf{1}_p \;|\; -\boldsymbol{\mu}, \boldsymbol{\Sigma} \right)$,

where

$\mathbf{1}_p = (1,\dots,1)^\mathsf{T} \in \mathbb{R}^p$

and

$G$ is the cdf of a multivariate normal distribution $\mathbf{X}$, that is to say,

$G(\mathbf{x}) = \mathrm{Pr}(\mathbf{X} \le \mathbf{x}) = \mathrm{Pr}(X_1 \le x_1 \;\;\text{and}\;\; \cdots \;\;\text{and}\;\; X_p \le x_p)$ .

I have tried to reproduce this result, but all I get is this:

$F(x) = \mathrm{Pr}(X_{(1)} \le x) = \mathrm{Pr}(X_1 \le x \;\;\text{or}\;\; \cdots \;\;\text{or}\;\; X_p \le x) = $

$1 - \mathrm{Pr}(X_1 > x \;\;\text{and}\;\; \cdots \;\;\text{and}\;\; X_p > x) =$

$1-\mathrm{Pr}(- X_1 < -x \;\;\text{and}\;\; \cdots \;\;\text{and}\;\; -X_p < -x)$,

and I do not know how to continue.

I know that, if $\mathbf{X} \sim \mathcal{N}(\boldsymbol{\mu}, \boldsymbol{\Sigma})$, then $-\mathbf{X} \sim \mathcal{N}(-\boldsymbol{\mu}, \boldsymbol{\Sigma})$. And this is all I am able to figure out.


EDIT:

I've been told that my question is already answered here, but I admit that I am not able to deduce the answer to my question from this other post, and that is why I would like to reopen my question.

Vicent
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  • This is turning into a FAQ -- but the previous versions are difficult to search for, since the question can be framed in so many equivalent ways. Kudos to @Xi'an for finding one of them. – whuber Apr 03 '21 at 12:52
  • @whuber Please, either reopen my question or let me know how my question was already solved. I don't get it... :/ – Vicent Apr 12 '21 at 18:01
  • Follow the link to the duplicate question. Note that all questions about minima of Normal variables are equivalent to questions about maxima of Normal variables because negating the variables converts one question into the other. – whuber Apr 12 '21 at 21:58
  • @whuber Yes, but I actually have problems to get from one point to the other one. Besides, in my question I showed a solution to my question that I found somewhere, but I am not sure of whether it is right or not, since I am not able to get to it on my own. – Vicent Apr 13 '21 at 06:07

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