Ext algebra of Nichols algebras of type $A_2$

Abstract

We give the full structure of the Ext algebra of a Nichols algebra of type $A_2$ by using the Hochschild-Serre spectral sequence. As an application, we show that the pointed Hopf algebras $u(\mathcal{D}, \lmd, \mu)$ with Dynkin diagrams of type $A$, $D$, or $E$, except for $A_1$ and $A_1\times A_1$ with the order $N_{J}>2$ for at least one component $J$, are wild.