Representation dimensions of triangular matrix algebras

Abstract: Let $A$ be a finite dimensional hereditary algebra over an algebraically closed field $k$, $T_2(A)=(\begin{array}{cc}A&0 A&A\end{array})$ be the triangular matrix algebra and $A^{(1)}=(\begin{array}{cc}A&0 DA&A\end{array})$ be the duplicated algebra of $A$ respectively. We prove that ${\rm rep.dim}\ T_2(A)$ is at most three if $A$ is Dynkin type and ${\rm rep.dim}\ T_2(A)$ is at most four if $A$ is not Dynkin type. Let $T$ be a tilting A-$\module$ and $\ol{T}=T\oplus\ol{P}$ be a tilting $A^{(1)}$-$\module$. We show that $\End_{A^{(1)}} \ol{T}$ is representation finite if and only if the full subcategory ${(X,Y,f)\ |\ X\in {\rm mod}\ A, Y\in\tau^{-1}\mathscr{F}(T_A)\cup{\rm add}\ A}$ of ${\rm mod \ T_2(A)}$ is of finite type, where $\tau$ is the Auslander-Reiten translation and $\mathscr{F}(T_A)$ is the torsion-free class of ${\rm mod}\ A$ associated with $T$. Moreover, we also prove that ${\rm rep.dim\ End}_{A^{(1)}}\ {\ol T}$ is at most three if $A$ is Dynkin type.