Let $(\mathfrak{ g}, [\cdot,\cdot], \delta_\mathfrak{ g})$ be a fixed Lie bialgebra, $E$ be a vector space containing $\mathfrak{ g}$ as a subspace and $V$ be a complement of $\mathfrak{ g}$ in $E$. A natural problem is that how to classify all Lie bialgebraic structures on $E$ such that $(\mathfrak{ g}, [\cdot,\cdot], \delta_\mathfrak{ g})$ is a Lie sub-bialgebra up to an isomorphism of Lie bialgebras whose restriction on $\mathfrak{ g}$ is the identity map. This problem is called the extending structures problem. In this paper, we introduce a general co-product on $E$, called the unified co-product of $(\mathfrak{ g},\delta_\mathfrak{ g})$ by $V$. With this unified co-product and the unified product of $(\mathfrak{ g}, [\cdot,\cdot])$ by $V$ developed in \cite{ AM1}, the unified bi-product of $(\mathfrak{ g}, [\cdot,\cdot], \delta_\mathfrak{ g})$ by $V$ is introduced. Moreover, we show that any $E$ in the extending structures problem is isomorphic to a unified bi-product of $(\mathfrak{ g}, [\cdot,\cdot], \delta_\mathfrak{ g})$ by $V$. Then an object $\mathcal{ HBI}_{ \mathfrak{ g}}^2(V,\mathfrak{ g})$ is constructed to classify all $E$ in the extending structures problem. Moreover, several special unified bi-products are also introduced. In particular, the unified bi-products when $\text{ dim} V=1$ are investigated in detail.
Categories: math.RA