Almost conservation of the harmonic actions for fully discretized nonlinear Klein–Gordon equations at low regularity

Abstract Close to the origin the nonlinear Klein–Gordon equations on the circle are nearly integrable Hamiltonian systems, which have infinitely many almost conserved quantities called harmonic actions or super-actions. We prove that, at low regularity and with a CFL number of size $1$, this property is preserved if we discretize the nonlinear Klein–Gordon equations with the symplectic mollified impulse methods. This extends previous results of D. Cohen, E. Hairer and C. Lubich to nonsmooth solutions.