In [the paper](https://www.brown.edu/academics/physics/sites/brown.edu.academics.physics/files/uploads/Parker-2015.pdf), (5.28) on page 35 is a formula which translate cluster A-coordinates to cluster X-coordinates: 
\begin{align}
x_i = \prod_{i \to j} a_j. 
\end{align}

Is there a formula which translate cluster X-coordinates to cluster A-coordinates? Thank you very much.

Edit: as David Speyer pointed out, the correction formula to translate cluster A-coordinates to cluster X-coordinates is
\begin{align}
x_i = \prod_{i \to j} a_j/\prod_{j \to i} a_j = \prod_{j} a_j^{Q_{ij}}.
\end{align}