We consider classical particles
on the line with the Weierstrass ℘ function as potential. This system parameterizes
special solutions of the KP equation. We derive the trace formula which
relates the Hamiltonian of the particle system to the residues of some Abelian
differential (meromorphic one-form) on the spectral curve. Such formula is
important for the construction action-angle variables and study invariant Gibbs’
states.