how many Q-forms of SL_n(R) are there for a given Q-rank - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-23T06:59:19Z http://mathoverflow.net/feeds/question/89777 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/89777/how-many-q-forms-of-sl-nr-are-there-for-a-given-q-rank how many Q-forms of SL_n(R) are there for a given Q-rank ronggang 2012-02-28T16:42:11Z 2013-01-12T11:17:30Z <p>Let $G$ be a linear algebraic group defined over $\mathbb Q$. Suppose that $G$ is isomorphic to $SL_n$ over $\mathbb R$. Suppose the $\mathbb Q$-rank of $G$ is fixed, say $m$. How many types are there for $G$ up to $\mathbb Q$-isomorphism? Are they finite especially for $m>2$?</p> http://mathoverflow.net/questions/89777/how-many-q-forms-of-sl-nr-are-there-for-a-given-q-rank/118722#118722 Answer by Aakumadula for how many Q-forms of SL_n(R) are there for a given Q-rank Aakumadula 2013-01-12T11:17:30Z 2013-01-12T11:17:30Z <p>I would refer to the very nice article by Jacques Tits in "Algebraic Groups and Discontinuous Subgroups" AMS Symposiain Pure Math Vol 9 (Boulder Conference) 1966. The title of the article by Tits is "Classification of Algebraic Groups", and gives the $k$-forms of simple algebraic groups over the separable algebraic closure of $k$ for any field $k$. </p> <p>From the tables in this article it is not difficult to deduce that the only $\mathbb Q$-forms of $SL_n$ which over $\mathbb R$ become isomorphic to $SL_n({\mathbb R})$ are the following</p> <p>(1) The groups $SL_m(D)$ where $D$ is a central division algebra over $\mathbb Q$ of degree $d$ such that $dm=n$ and $D\otimes {\mathbb R}=M_2({\mathbb R})$. As Keerti Madapusi has observed, there are infinitely many such. </p> <p>(2) The groups $SU_m(D,h)$ where $K/{\mathbb Q}$ is a real quadratic extension, $D$ is a central division algebra over $E$ with an involution of the second kind such that the involution restricted to $E/Q$ is the non-trivial element of the Galois group of $E/{\mathbb Q}$, and $h:D^m\times D^m \rightarrow D$ is Hermitian with respect to this involution. Furthermore, $D\otimes E_v$ is the matrix algebra $M_2(E_v)$ for both the archimedean embeddings $E_v$ of $E$. </p> <p>I think these may even be worked out in Dave Witte's e-book on arithmetic groups (not very sure). </p>