Trade Empire Core Strength (manual)

Trade drops off less sharply than Culture, but produces a smaller yield. It has the potential to put a system in the connected empire relationship (which increases your trade potential, reduces the system's force cost, and gives you some wealth).

At the time of this writing, the formula for trade is as follows:

$$\sqrt[{4 \over 3}]{A \times \sqrt[3]{W} }$$