Semi-derived Ringel-Hall bialgebras

Let $\mathcal{A}$ be an arbitrary hereditary abelian category. Lu and Peng defined the semi-derived Ringel-Hall algebra $SH(\mathcal{A})$ of $\mathcal{A}$ and proved that $SH(\mathcal{A})$ has a natural basis and is isomorphic to the Drinfeld double Ringel-Hall algebra of $\mathcal{A}$. In this paper, we introduce a coproduct formula on $SH(\mathcal{A})$ with respect to the basis of $SH(\mathcal{A})$ and prove that this coproduct is compatible with the product of $SH(\mathcal{A})$, thereby the semi-derived Ringel-Hall algebra of $\mathcal{A}$ is endowed with a bialgebra structure which is identified with the bialgebra structure of the Drinfeld double Ringel-Hall algebra of $\mathcal{A}$.