\begin{notation}
    Let $\cB_n$ denote $\cB(\R^n)$.
\end{notation}
\begin{goal}
    Suppose we have a probability measure $\mu_n$ on $(\R^n, \cB(\R^n))$
    for each $n$.
    We want to show that there exists a unique probability measure $\bP^{\otimes}$
    on $(\R^\infty, \cB_\infty)$ (where $\cB_{\infty}$ still needs to be defined),
    such that $\bP^{\otimes}\left( \prod_{n \in \N}  B_n \right) = \prod_{n \in \N} \mu_n(B_n)$
    for all $\{B_n\}_{n \in  \N}$, $B_n \in \cB_1$.
\end{goal}
% $\bP_n = \mu_1 \otimes \ldots \otimes \mu_n$.
\begin{remark}
    $\prod_{n \in \N} \mu_n(B_n)$ converges, since $0 \le \mu_n(B_n) \le 1$
    for all $n$.
\end{remark}

First we need to define $\cB_{\infty}$.
This $\sigma$-algebra must contain all sets $\prod_{n \in \N} B_n$
for all $B_n \in  \cB_1$. We simply define $\cB_{\infty}$ to be the
$\sigma$-algebra
Let $\cB_\infty \coloneqq  \sigma \left( \{\prod_{n \in \N} B_n | B_n \in  \cB(\R)\}  \right)$.
\begin{question}
    What is there in $\cB_\infty$?
    Can we identify sets in $\cB_\infty$ for which we can define the product measure
    easily?
\end{question}
Let $\cF_n \coloneqq  \{ C \times \R^{\infty} | C \in \cB_n\}$.
It is easy to see that $\cF_n \subseteq  \cF_{n+1}$
and using that $\cB_n$ is a $\sigma$-algebra, we can show that $\cF_n$
is also a $\sigma$-algebra.
Now, for any $C \subseteq  \R^n$ let $C^\ast \coloneqq  C \times \R^{\infty}$.

Thus $\cF_n = \{C^\ast : C \in  \cB_n\}$.
Define $\lambda_n : \cF_n : \to [0,1]$ by $\lambda_n(C^\ast) \coloneqq  (\mu_1 \otimes \ldots \otimes \mu_n)(C)$.
It is easy to see that $\lambda_{n+1} \defon{\cF_n} = \lambda_n$ (\vocab{consistency}).


Recall the following theorem from measure theory:
\begin{theorem}[Caratheodory's extension theorem] % 2.3.3 in the notes
    \label{caratheodory}
    Suppose $\cA$ is an algebra (i.e.~closed under finite union)
    und $\Omega \neq  \emptyset$.
    Suppose $\bP$ is countably additive on $\cA$ (i.e.~if $(A_n)_{n}$ 
    are pairwise disjoint and $\bigcup_{n \in \N} A_n \subseteq  \cA $
    then $\bP\left( \bigcup_{n \in \N} A_n \right) = \sum_{n \in \N}  \bP(A_n)$).
    Then $\bP$ extends uniquely to a probability measure on $(\Omega, \cF)$,
    where $\cF = \sigma(\cA)$.
\end{theorem}

Define $\cF = \bigcup_{n \in \N} \cF_n$. Check that $\cF$ is an algebra.
We'll show that if we define $\lambda: \cF \to  [0,1]$ with
$\lambda(A) = \lambda_n(A)$ for any $n$ where this is well defined,
then $\lambda$ is countably additive on $\cF$.
Using \autoref{caratheodory}  $\lambda$ will extend uniquely to a probability measure on $\sigma(\cF)$.

We want to prove:
\begin{enumerate}[(1)]
    \item $\sigma(\cF) = \cB_\infty$,
    \item $\lambda$ as defined above is countably additive on $\F$.
\end{enumerate}
\begin{proof}[Proof of (1)]
    Consider an infinite dimensional box  $\prod_{n \in \N} B_n$.
    We have
     \[
         \left( \prod_{n=1}^N B_n \right)^\ast \in \cF_n \subseteq \cF
    \]
    thus
    \[
    \prod_{n \in \N} B_n = \bigcap_{N \in \N} \left( \prod_{n=1}^N B_n \right)^\ast \in \sigma(\cF).
    \]
    Since $\sigma(\cF)$ is a $\sigma$-algebra, $\cB_\infty \subseteq  \sigma(\cF)$. This proves ``$\supseteq$''.
    For the other direction we'll show $\cF_n \subseteq \cB_\infty$ for all $n$.
    Let  $\cC \coloneqq  \{ Q  \in  \cB_n | Q^\ast \in \cB_\infty\}$.
    For $B_1,\ldots,B_n \in \cB$, $B_1 \times \ldots \times  B_n \in \cB_n$
    and $(B_1 \times \ldots \times B_n)^\ast \in \cB_\infty$.
    We have $B_1 \times  \ldots \times  B_n \in  \cC$.
    And $\cC$ is a $\sigma$-algebra, because:
    \begin{itemize}
        \item $\cB_n$ is a $\sigma$-algebra
        \item  $\cB_\infty$ is a $\sigma$-algebra,
        \item $\phi^\ast  \phi$, $(\R^n \setminus Q)^\ast = \R^{\infty} \setminus Q^\ast$, $\bigcup_{i \in I} Q_i^\ast = (\bigcup_{i \in I} Q_i)^\ast$.
    \end{itemize}
    Thus $\cC \subseteq  \cB_n$ is a $\sigma$-algebra and contains all rectangles, hence $\cC = \cB_n$.
    Hence  $\cF_n \subseteq  \cB_\infty$ for all $n$,
    thus $\cF \subseteq  \cB_\infty$. Since $\cB_\infty$ is a $\sigma$-algebra,
    $\sigma(\cF) \subseteq  \cB_\infty$.
\end{proof}
We are going to use the following
\begin{fact}
    \label{fact:finaddtocountadd}
    Suppose $\cA$ is an algebra on $\Omega \neq \emptyset$,
    and suppose $\bP: \cA \to [0,1]$ is a finitely additive
    probability measure.
    Suppose whenever $\{B_n\}_n$ is a sequence of sets from $\cA $
    decreasing to $\emptyset$ it is the case that
    $\bP(B_n) \to 0$. Then $\bP$ must be countably additive.
\end{fact}
\begin{proof}
    Exercise
    
\end{proof}
\begin{proof}[Proof of (2)]
    Let's prove that $\lambda$ is finitely additive.
    $\lambda(\R^\infty) = \lambda_1(\R^\infty) = 1$.
    $\lambda(\emptyset) = \lambda_1(\emptyset) = 0$.
    Suppose  $A_1, A_2 \in \cF$ are disjoint.
    Then pick some $n$ such that $A_1, A_2 \in \cF_n$.
    Take $C_1, C_2 \in \cB_n$ such that $C_1^\ast = A_1$
    and $C_2^\ast = A_2$.
    Then $C_1$ and $C_2$ are disjoint and $A_1 \cup  A_2 = (C_1 \cup  C_2)^\ast$.
    $\lambda(A_1 \cup  A_2) = \lambda_n(A_1 \cup  A_2) = (\mu_1 \otimes \ldots \otimes \mu_n)(C_1 \cup  C_2) = \lambda_n(C_1) + \lambda_n(C_2)$
    by the definition of the finite product measure.

    In order to use \autoref{fact:finaddtocountadd},
    we need to show that if $B_n \in \cF$ with $B_n \to \emptyset \implies \lambda(B_n) \to 0$.
    %TODO
\end{proof}