2024/07/29 by Raphael Appenzeller, Appenzeller, Raphael
Mathematics · #12J15 #14P10 #20E42 #20G25 #Advanced Topics in Algebra #FOS: Mathematics #Group Theory (math.GR) #Homotopy and Cohomology in Algebraic Topology
paper · pdf · doi:10.48550/arxiv.2407.20406
openalex publication_date 2024/07/29 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
We develop the theory of algebraic groups over real closed fields and apply the results to construct a geometric object B and to prove that B is an affine Λ-building. We use a model theoretic transfer principle to prove generalizations of statements about semisimple Lie groups. In this direction we give proofs for the Iwasawa-decomposition KAU, the Cartan-decomposition KAK and the Bruhat-decomposition BWB. For unipotent subgroups we prove the Baker-Campbell-Hausdorff formula and use it to analyse root groups. We give a proof of the Jacobson-Morozov Lemma about subgroups whose Lie algebra is isomorphic to \mathfraksl2 and we describe other rank 1 subgroups which are the semisimple parts of Levi-subgroups. We prove a semialgebraic version of Kostant's convexity. Over the reals, semisimple Lie groups are closely related to the symmetry groups of symmetric spaces of non-compact type. These symmetric spaces can be described semialgebraically, which allows us to consider their semialgebraic extension over any real closed field. Starting from these non-standard symmetric spaces we use a valuation (with image some non-discrete ordered abelian group Λ) on the fields to define a Λ-pseudometric. Identifying points of distance zero results in a Λ-metric space B. Assuming that the root system of the associated Lie group is reduced, we prove that B is an affine Λ-building. The proof relies on a thorough analysis of stabilizers.