Coideal subalgebras of pointed and connected Hopf algebras

Abstract

Let $H$ be a pointed Hopf algebra with abelian coradical. Let $A\supseteq B$ be left (or right) coideal subalgebras of $H$ that contain the coradical of $H$. We show that $A$ has a PBW basis over $B$, provided that $H$ satisfies certain mild conditions. In the case that $H$ is a connected graded Hopf algebra of characteristic zero and $A$ and $B$ are both homogeneous of finite Gelfand-Kirillov dimension, we show that $A$ is a graded iterated Ore extension of $B$. These results turn out to be conceptual consequences of a structure theorem for each pair $S\supseteq T$ of homogeneous coideal subalgebras of a connected graded braided bialgebra $R$ with braiding satisfying certain mild conditions. The structure theorem claims the existence of a well-behaved PBW basis of $S$ over $T$. The approach to the structure theorem is constructive by means of a combinatorial method based on Lyndon words and braided commutators, which is originally developed by V. K. Kharchenko for primitively generated braided Hopf algebras of diagonal type. Since in our context we don't priorilly assume $R$ to be primitively generated, new methods and ideas are introduced to handle the corresponding difficulties, among others.