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Integrable properties of the differential-difference Kadomtsev-Petviashvili hierarchy and continuum limits

The paper reveals clear links between the differential-difference Kadomtsev-Petviashvili hierarchy and the (continuous) Kadomtsev-Petviashvili hierarchy, together with their symmetries, Hamiltonian structures and conserved quantities. They are connected through a uniform continuum limit. For the differential-difference Kadomtsev-Petviashvili system, we introduce Lax triads to generate isospectral and non-isospectral flows. This approach provides an integrable master symmetry and simple zero curvature representations of flows. The obtained flows are then proved to generate a Lie algebra w.r.t. Lie product ${\llbracket} \cdot, \cdot {\rrbracket}$, which leads to two sets of symmetries for the isospectral differential-difference Kadomtsev-Petviashvili hierarchy, and the symmetries generate a Lie algebra, too. The algebra of flows also provide recursive relations of the flows via the master symmetry, which are then used to derive Hamiltonian structures for both isospectral and non-isospectral differential-difference Kadomtsev-Petviashvili hierarchies. The Hamiltonians generate a Lie algebra w.r.t. the Poisson bracket $\{\cdot,\cdot\}$. The Hamiltonians together with symmetries lead to two sets of conserved quantities for the whole isospectral differential-difference Kadomtsev-Petviashvili hierarchy and they also generate a Lie algebra. All the obtained algebras have same basic structures. Then, we provide a continuum limit which is different from Miwa's transformation. By means of defining \textit{degrees} of some elements with respect to the continuum limit, we find that the differential-difference Kadomtsev-Petviashvili hierarchies together with their Lax triads, zero curvature representations and all integrable characteristics go to their continuous counterparts in the continuum limit. We also explain the basic structure deformation of Lie algebras in the continuum limit.