The rule of the game is to use A and F=dA to write a topological action, and in d+1-space time dimension you need to come up with a gauge-invariant d+1-form which can then be integrated over the manifold to give you the action. Such an action does not depend on metric at all. Take U(1) gauge field as an example. In 2+1, the only thing you can write down is AF(AAA vanishes identically), which is the Chern-Simons. Then in 3+1, you can guess FF,AAF,AAAA. AAF and AAAA vanishes due to antisymmetrization of the wedge product. So you are left with FF. This can be generalized to other Lie groups.
This post imported from StackExchange Physics at 2015-03-21 18:34 (UTC), posted by SE-user Meng Cheng