# Dynkin diagram of the centralizer of a semisimple element in a Levi subgroup

Let $G$ be a connected reductive group over an algebraically closed field and consider a semisimple element $s \in G$ and let $L$ be a Levi subgroup containing $s$.

My question is about the two ways we can look at $C_L(s)^\circ$. On the one hand, it is the connected centralizer of $s$ in the Levi subgroup $L$, on the other hand, it is a Levi subgroup in the connected centralizer $C_G(s)^\circ$.

The Dynkin diagram of $C_{G}(s)^\circ$ can be obtained from the extended Dynkin diagram of $G$ by removing nodes. By removing appropriate nodes from the Dynkin diagram of $C_{G}(s)^\circ$, we obtain the Dynkin diagram of the Levi subgroup $C_{L}(s)^\circ$ of $C_{G}(s)^\circ$.

On the other hand, the Dynkin diagram of $C_L(s)^\circ$ can be obtained from the extended Dynkin diagram of $L$ and the Dynkin diagram of $L$ can be obtained by removing nodes from the Dynkin diagram of $G$.

For example let $G$ have type $B_{10}$ and choose $s$ such that $C_{G}(s)^\circ$ has type $D_4 \times B_6$, then I can find a Levi subgroup of $C_{G}(s)^\circ$ of type $D_4 \times A_2 \times B_3$. This Levi is of the form $C_L(s)^\circ$ for some Levi $L$ of $G$.

However, even though there exists a Levi subgroup of $G$ of type $A_2 \times B_7$ and one of its subgroups of maximal rank has the desired type $A_2 \times D_4 \times B_3$, the bases of the corresponding root systems seem to be completely different to me. In fact, I am not even sure if that subgroup of type $A_2 \times D_4 \times B_3$ lies in $C_G(s)$ or not.

Also, it seems to me that the other Levi subgroups of $G$ do not even have a subgroup of the desired type.

Did I do anything wrong here? If not, why is it not possible for me to obtain the same basis of the Levi of $C_{G}(s)^\circ$ when considering it as a maximal rank subgroup of a Levi subgroup of $G$?

• I'm having a little trouble with the question. Are you asking why it's not the case that a specific $D_4 \times A_2 \times B_3$ subgroup of $B_{10}$ embeds in both a $D_4 \times B_6$ and an $A_2 \times B_7$ subgroup? Is there any reason to expect that it would? Mar 17, 2017 at 17:14
• @LSpice : To clarify my question, I have a Levi of $C_G(s)^\circ$ of type $D_4 \times A_2 \times B_3$, I would like to know how I have to choose $L$ so that this Levi is $C_L(s)^\circ$. You seem to have a suggestion for $L$, however, I do not see how removing nodes from the extended Dynkin diagram of type $A_6 \times B_3$ could give me type $A_2 \times D_4 \times B_3$. Mar 17, 2017 at 19:11
• My $L$ was total nonsense (wrong type, and wrong realisation of that type!). I have posted an answer that I think shows that your expectations are correct, but that you have to conjugate your choice of $L$ (or of $s$) a little to realise them. Mar 17, 2017 at 20:01
• @JimHumphreys, $C_G(s)^\circ \cap L = C_G(s)^\circ \cap C_G(Z(L)^\circ)$ is the centraliser in $C_G(s)^\circ$ of the torus $Z(L)^\circ$, and the torus lies in $C_G(s)^\circ$ since $s$ lies in $L$; so the intersection is connected, hence equals $C_L(s)^\circ$, which is therefore a Levi subgroup of $C_G(s)^\circ$. Mar 20, 2017 at 1:59
• @JimHumphreys: There is no need to associate a $\lambda$ to the semisimple element $s$. The torus to be considered is $Z(L)^\circ$. Its centralizer in $G$ is a Levi subgroup of $G$, namely $L$, and its centralizer in $C_G(s)^\circ$ is also a Levi subgroup of $C_G(s)^\circ$. By the above argument, it turns out to be $C_L(s)^\circ$. Mar 21, 2017 at 6:25

In the Bourbaki numbering ($\alpha_i = \epsilon_i - \epsilon_{i + 1}$ for $i < 10$ and $\alpha_{10} = \epsilon_{10}$), I believe that you can take $s = \alpha_1^\vee(-1)\alpha_3^\vee(-1)$ (with connected centraliser of type $D_4 \times B_6$, where the base for the $D_4$ piece is $\{\alpha_3, \alpha_2, \alpha_1, -\mu\}$ and that for the $B_6$ piece is $\{\alpha_5, \dotsc, \alpha_{10}\}$) and $L$ to be the centraliser of the image of $\alpha_5^\vee + 2\alpha_6^\vee + 3(\alpha_7^\vee + \dotsb + \alpha_{10}^\vee)$, which, as you predicted, is of type $A_2 \times B_7$. The base for the $A_2$ piece is $\{\alpha_5, \alpha_6\}$, and that for the $B_7$ piece is $\{\alpha_1, \alpha_2, \alpha_3, \alpha_4 + \dotsb + \alpha_7, \alpha_8, \alpha_9, \alpha_{10}\}$.
• @MatthiasKlupsch, I don't mean to be argumentative, but I would argue that "manipulating the Dynkin diagram as a graph" is just what your post did: using just the graph, you were able to determine the types of $C_G(s)^\circ$, $L$, and $C_L(s)^\circ$. It was only when it came to writing down a particular choice of basis that you were forced to deal with information beyond the root diagram; but, as you say, as soon as you attempt to do that you are attaching extra information to the nodes and edges, and it is perhaps no surprise that you have to do something with that information. Mar 20, 2017 at 21:57
• Yes, you are right, this is what I was trying to do and I was able to determine the type of $L$ given the one of $C_{L}(s)^circ$ in this example. But this was only because I could look at all possible types of Levi subgroups and see that there was only one of them where the type of $C_{L}(s)^circ$ could occur. In general, these manipulations only seem to find candidates for the types of Levi subgroups. I believe I just had my hopes too high. Mar 24, 2017 at 8:34