# On Borel subalgebras of quantum groups

## TL;DR

This paper classifies and analyzes Borel subalgebras of quantum groups, revealing new types and their module structures, with explicit classifications for low-rank cases and conjectures on their general shape.

## Contribution

It introduces the concept of Borel subalgebras in quantum groups, identifies new examples, and studies their module theory and structural properties, including conjectures on their classification.

## Key findings

- Identified all Borel subalgebras of U_q(sl_2) and U_q(sl_3).
- Proved that induced modules from Borel subalgebras have all finite-dimensional irreducible modules as quotients.
- Proposed and proved structural conjectures for the shape of triangular Borel subalgebras.

## Abstract

For a quantum group, we study those right coideal subalgebras, for which all irreducible representations are one-dimensional. If a right coideal subalgebra is maximal with this property, then we call it a Borel subalgebra. Besides the positive part of the quantum group and its reflections, we find new unfamiliar Borel subalgebras, for example, ones containing copies of the quantum Weyl algebra. Given a Borel subalgebra, we study its induced (Verma-)modules and prove among others that they have all irreducible finite-dimensional modules as quotients. We give two structural conjectures involving the associated graded right coideal subalgebra, which we prove in certain cases. In particular, they predict the shape of all triangular Borel subalgebras. As examples, we determine all Borel subalgebras of $U_q(\mathfrak{sl}_2)$ and $U_q(\mathfrak{sl}_3)$ and discuss the induced modules.