On irreducible morphisms and Auslander-Reiten triangles in the stable category of modules over repetitive algebras

Let $mathbf{k}$ be an algebraically closed field, let $Lambda$ be a finite dimensional $mathbf{k}$-algebra, and let $widehat{Lambda}$ be the repetitive algebra of $Lambda$. For the stable category of finitely generated left $widehat{Lambda}$-modules $widehat{Lambda}$-underline{mod}, we show that the irreducible morphisms fall into three canonical forms: (i) all the component morphisms are split monomorphisms; (ii) all of them are split epimorphisms; (iii) there is exactly one irreducible component. We next use this fact in order to describe the shape of the Auslander-Reiten triangles in $widehat{Lambda}$-underline{mod}. We use the fact (and prove) that every Auslander-Reiten triangle in $widehat{Lambda}$-underline{mod} is induced from an Auslander-Reiten sequence of finitely generated left $widehat{Lambda}$-modules.