# How to do the classification of Yangian invariant?

From Nima's paper:Scattering Amplitudes and the Positive Grassmannian arxiv:1212.5605 page91, we can see that there is a complete classification for $$k=2$$ Yangian inariants. But I have two questions

1. How to get the Yangian from the matrix $$C$$? I think I should solve the equation $$$$C\cdot Z=0$$$$ where $$C\in G_{+}(2,n)$$ and $$Z\in M_{n,4}$$ which is the external data for $$n$$ particles in 4 dimension.

But when I use mathematica to solve these equation, the solution is absolutely wrong. I think the solution $$\alpha$$ is supposed to be the ratio of Plucker. Yet it's not.

1. How do one know that there's only 14 cyclic distinct Yangian? How do we get the enumerate all the permutation and select out the right one?