# A characterization of distinguished triangles in triangulated categories.

Let $\mathscr{D}$ be a triangulated category. Let $$X \longrightarrow Y \longrightarrow Z \longrightarrow X[1]$$ be a triangle (not necessarily distinguished). We call it special if for each $E \in \mathscr{D}$ the induced long sequence of abelian groups $$\cdots Hom(E,X) \rightarrow Hom(E,Y) \rightarrow Hom(E,Z) \rightarrow Hom(E,X[1]) \cdots$$ is exact. Since for each $E \in \mathscr{D}$ the functor $Hom(E,-)$ is homological, distinguished triangles are special. But is it true that special triangles are distinguished? Thank you very much.