# Representation theory, classical Lie algebra, D_{n}

I want to know the fundamental representation of classical Lie algebra of type $D_{n}$ over complex numbers with the following informations. For example, $L(\omega_i)$ be a fundamental rep of fundamental weight $\omega_i$, supposed $i\neq 1$(easy) and $i\neq n$(no idea what the rep looks like for now, and I am reading it). We know $L(\omega_i)\cong \bigwedge^{i}\mathbb{C}^{2n}$.

1. What is a highest weight vector, and a lowest weight vector? The highest one seems to be $e_1\wedge\ldots\wedge e_i$, where $e_k$ is a standard basis of C^{2n}?

2. If possible to apply simple root vectors to highest weight vector to obtain the lowest weight vector($v^{-}=X_{\alpha_{i_1}}^{-}\ldots X_{\alpha_{i_k}}^{-}v^{+}$)? If so, do you know the rules?

3. If it is possible to answer same question for $i=n$?

Thank you so much.

• What do you mean by applying root vectors to highest weight vectors? Do you mean applying the corresponding elements in the Weyl group? Sep 12 '13 at 8:09
• Your notation is not the standard one. $D_{2n}$ acts on ${\mathbb C}^{4n}$. So you must replace $D_{2n}$ with $D_n$ (the $n$ here refers to "rank" not to the dimension of the standard representation). Sep 12 '13 at 10:19
• Secondly, to specify a highest weight vector, you must fix a Borel subalgebra. If the quadratic form is chosen suitably, then the Borel subalgebra is just the algebra of upper triangular matrices in $SO(2n,{\mathbb C})$. Then $e_1\wedge \cdots \wedge e_i$ is indeed the highest weight vector. And its translate by the longest weyl group element is the lowest weight vector. All this is standard and you may look up Fulton-Harris, or Humphreys' book on Lie algebras. Sep 12 '13 at 10:23

First, note that your edited question still has $i=2n$ when you mean $i=n$. As was pointed out by others, $n$ is the rank here and $2n$ the dimension of the first fundamental representation (the natural one for the Lie algebra in the even orthogonal case).

One useful source (if you can locate it), based on lectures given by J. Tits to mathematical physicists in Bonn many years ago, is his Springer Lecture Notes No. 40 (1967) with a German title Tabellen zu den einfachen Lie Gruppen und ihren Darstellungen [Tables for the simple Lie groups and their representations]. But the tables toward the end of this short volume involve little German (and Tits himself is Belgian/French). He provides explicit data including dimensions of all the fundamental representations, describing also the results for the various real forms.

In particular, the two end vertices of the Dynkin diagram usually labelled $n-1$ and $n$, correspond to the half-spin representations of the Lie algebra. Each has dimension $2^{n-1}$. They are not so easily constructed in terms of exterior powers of the natural representation as the earlier fundamental representations, however, with a digression into Clifford algebras usually required.

ADDED: Note that the last statement in your first paragraph is still incorrect. When you write "We know ..." this applies only for $i \leq n-2$ whereas for $n-1, n$ you get half-spin modules as discussed by Carter in his section 13.5. (This kind of concrete description of representations is found in quite a few texts.)

Concerning your questions 2, 3, it's of course possible to reach any weight vector from the highest one by subtracting various simple (or arbitrary positive) roots: By ordering the negative roots and using PBW monomials in negative root vectors, you can produce any weight space including the lowest one. The problem is that many monomials typically produce linearly dependent weight vectors, which is what makes the character theory complicated.

In particular, for $i \leq n-2$ (or for $n$ even) all fundamental representations are self-dual and therefore the lowest weight is just the negative of the highest weight under the action of the longest element $w_\circ$ of the Weyl group. (For $n$ odd, $w_\circ$ is not $-1$, so each of the last two fundamental weights goes to the negative of the other.) From the standard tables you can write down the difference between highest and lowest weights as a $\mathbb{Z}$-linear combination of simple (or arbitrary positive) roots, then construct various PBW monomials taking a highest weight vector to a lowest weight vector. Not very informative.

• I changed the mistakes. a book of R.W.Carter, Lie algebras of finite and affine type, give explicit constructions on fundamental representation of simple lie algebra in Chapter 13.<br/> Can you offer a little comment on question 2? Sep 19 '13 at 17:11

Maybe the following article will be helpful: http://arxiv.org/abs/math/9902060 (A. I. Molev, A weight basis for representations of even orthogonal Lie algebras).

For a quick and elementary introduction into the representation theory, see a little book: http://archive.org/details/GroupTheoryElementaryParticles (Group Theory and Elementary Particles, by Penelope A. Rowlatt).