Spectrum transformation and conservation laws of the lattice potential KdV equation

Many multi-dimensional consistent discrete systems have soliton solutions with nonzero backgrounds, which brings difficulty in the investigation of integrable characteristics. In this letter we derive infinitely many conserved quantities for the lattice potential Korteweg-de Vries equation. The derivation is based on the fact that the scattering data $a(z)$ is independent of discrete space and time and the analytic property of Jost solutions of the discrete Schr\"odinger spectral problem. The obtained conserved densities are different from those in the known literatures. They are asymptotic to zero when $|n|$ (or $|m|$) tends to infinity. To obtain these results, we reconstruct a discrete Riccati equation by using a conformal map which transforms the upper complex plane to the inside of unit circle. Series solution to the Riccati equation is constructed based on the analytic and asymptotic properties of Jost solutions.