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Tuesday, September 27, 2016

Weight diagrams of G_2

This post contains pictures of weight diagrams of irreducible representations of the Lie algebra G_2.
The dimensions of the irreducible representations of G_2 are [1] \begin{equation*} 1, 7, 14, 27, 64, \ldots \end{equation*} Cahn [2] explains how to calculate the weights of representations and how to use Freudenthal's formula to calculate the dimensions of the weight spaces. I programmed these formulas in Mathematica to obtain the figures below.
The 7-dimensional representation
The Dynkin labels of the highest weight are | 0, 1 \rangle. The list of all weights in \bf{7} is | 0, 1 \rangle, | 1,-1 \rangle, | -1,2\rangle, | 0,0\rangle, | 1,-2\rangle, | -1,1\rangle and | 0, -1\rangle. All weight spaces are 1-dimensional. If I plot them on the root diagram, I get the following picture
The weights of the 7-dimensional representation of G_2.
The black arrows are the root vectors.

The 14-dimensional representation
The 14-dimensional representation of G_2 is the adjoint representation, the weights thus coincide with the roots. The highest weight is |1,0\rangle. The weight space of |0,0\rangle is 2-dimensional, because the Cartan subalgebra is 2-dimensional. All other weight spaces are 1-dimensional.
The weights of the 14-dimensional representation of G_2

The 27-dimensional representation
This is the representation with highest weight |0,2\rangle
The weights of the 27-dimensional representation of G_2
In this case, the dimension of the weight spaces can be 1, 2 or 3-dimensional. In the next figure, I have plotted the dimension of the weight space at the corresponding weight.
The dimensions of the weight spaces of the 27-dimensional representation of G_2

The 64-dimensional representation
This is the representation with highest weight |1,1\rangle. I only plot the dimensions of the weight spaces.
The dimensions of the weight spaces of the 64-dimensional representation of G_2
All these pictures look quite similar, so I will only make one more. Here is the weight diagram of the 1547-dimensional representation. This is the representation with highest weight |2,3\rangle
The dimensions of the weight spaces of the 1547-dimensional representation of G_2
I was inspired to make these weight diagrams by a webpage from Bump [3].
References
[1] Wikipedia article
[2] Cahn, Semi-Simple Lie Algebras and Their Representations, Chapters X and XI
[3] Daniel Bump, Weight Diagrams of Weight Two

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