Understanding the Osterwalder-Schrader conditions as formulated by Glimm and Jaffe $\newcommand{\real}{\mathrm{real}}$I am having trouble with understanding the axiom (OS3) in this book by Glimm and Jaffe.
It defines
\begin{equation}
\mathcal{A} = \left \{ A(\phi) = \sum_{j = 1}^N c_j \exp \left( \phi(f_j) \right)  \; \Big \vert \; c_j \in \mathbb{C}, f_j \in \mathcal{D} \right \}
\end{equation}
where $\mathcal{D}$ is supposed to be the space of compactly supported, smooth functions on $\mathbb{R}^d$ and $\phi \in \mathcal{D}'(\mathbb{R}^d)$.
Now, this is notationally somewhat messed up (imho) but it is also stated earlier in the text, that $\mathcal{D}'(\mathbb{R}^d)$ should refer to real distributions hence, I assume that $\mathcal{D}$ should also refer to real functions - but I have not found this statement anywhere.
They then go on by defining the time-reflection operator $\theta : \mathcal{A} \to \mathcal{A}$ and consider a special supspace $\mathcal{A}_+ \subset \mathcal{A}$ and give the axiom:
\begin{equation}
0 \le \int_{\mathcal{D}'(\mathbb{R}^d)} \left( \theta A \right)^- A \mathrm{d} \mu
\end{equation}
where the superscript $-$ is supposed to mean complex conjugation.
This might sound okay but then they state that this is equivalent to the positive definiteness of matrices of the form
\begin{equation}
M_{i j} = \int_{\mathcal{D}'(\mathbb{R}^d)} \exp \left[ i \phi \left( f_i - \theta f_j \right) \right] \mathrm{d} \mu \left( \phi \right)
\end{equation}
with $f_j$ in the new space $\mathcal{D}_{\real}(\mathbb{R}^d)$.
I feel that it is completely unclear whether they are talking about $\mathcal{D}$ as a space of real or complex functions.
Furthermore, I cannot see the equivalence of the two statements unless $\mathcal{A}$ was defined with an additional $i$ in the exponent and with $\mathcal{D} \supset \mathcal{D}_{\real}(\mathbb{R}^d)$ as a space of real functions.
nlab also gives the almost identical definition as the book but without stating anything about real or complex spaces.
 A: Since this has confused me multiple times, I write this answer in the hope that it might help others.
First, recall that reflection positivity as formulated by Osterwalder and Schrader states that
\begin{equation}
\sum_{n,m} G_{n+m} \left( \theta f_n^* \otimes f_m \right) \ge 0
\end{equation}
for all finite sequences $(f_n)_{n \in \mathbb{N}}$ of complex Schwartz functions on $(\mathbb{R}^d)^n$ with supports in $(\mathbb{R}_{\ge 0} \times \mathbb{R}^{d-1})^n$.
Here,
\begin{equation}
\left( \theta f \right) \left( x^1_1, \dots, x^1_d, x^2_1, \dots x^d_d \right)
=
f \left( -x^1_1, x^1_2 \dots, x^1_d, -x^2_1, x^2_2 \dots x^2_d, \dots \right)
\end{equation}
for all $x^1, \dots x^d \in \mathbb{R}^d$.
Second, the whole idea of having a measure $\mu$ on a space of distributions is to be able to write the $G_n$s as moments of that measure.
From here on $\mu$ will be a corresponding measure on the space of real distributions.
Also, let $Z$ denote the moment-generating function of $\mu$ (assuming it exists) and $\hat{\mu}$ its characteristic function.
Now, for any finite sequences $(\phi_n, c_n)_{n \in \mathbb{N}}$ of real Schwartz functions on $\mathbb{R}^d$ with supports on $\mathbb{R}_{\ge 0} \times \mathbb{R}^{d-1}$ and complex numbers, define
\begin{equation}
f_a = \frac{1}{a!} \sum_n c_n \phi_n^{\otimes a}
\end{equation}
for all $a \in \mathbb{N}$.
Then a simple calculation (assuming everything is well-defined) shows that
\begin{equation}
\sum_{m, n} c_n^* c_m Z \left( \theta \phi_n + \phi_m \right)
=
\sum_{a, b = 0}^\infty G_{a+b} \left( \theta f_a^* \otimes f_b \right)
=
\lim_{N \to \infty} \sum_{a, b = 0}^N G_{a+b} \left( \theta f_a^* \otimes f_b \right)
\ge 0 \, .
\end{equation}
Similarly, we obtain a connection with the characteristic function by setting
\begin{equation}
g_a = \frac{\left(-i\right)^a}{a!} \sum_n c_n \phi_n^{\otimes a}
\end{equation}
for all $a \in \mathbb{N}$.
Then
\begin{equation}
\sum_{m, n} c_n^* c_m \hat{\mu} \left( \theta \phi_n - \phi_m \right)
=
\sum_{a, b = 0}^\infty G_{a+b} \left( \theta g_a^* \otimes g_b \right)
=
\lim_{N \to \infty} \sum_{a, b = 0}^N G_{a+b} \left( \theta g_a^* \otimes g_b \right)
\ge 0 \, .
\end{equation}
Observe that we need either $Z( \theta \phi_n + \phi_m )$ or $\hat{\mu}( \theta \phi_n - \phi_m )$.
