EN
We discuss invariants and conservation laws for a nonlinear system of Klein-Gordon equations with Hamiltonian structure
⎧$u_{tt} - Δu + m²u = -F₁(|u|²,|v|²)u$,
⎨
⎩$v_{tt} - Δv + m²v = -F₂(|u|²,|v|²)v$
for which there exists a function F(λ,μ) such that
∂F(λ,μ)/∂λ = F₁(λ,μ), ∂F(λ,μ)/∂μ = F₂(λ,μ).
Based on Morawetz-type identity, we prove that solutions to the above system decay to zero in local L²-norm, and local energy also decays to zero if the initial energy satisfies
$E(u,v,ℝⁿ,0) = 1/2 ∫_{ℝⁿ}(|∇u(0)|² + |u_t(0)|² + m²|u(0)|² + |∇v(0)|² + |v_t(0)|² + m²|v(0)|² + F(|u(0)|²,|v(0)|²))dx < ∞$,
and
F₁(|u|²,|v|²)|u|² + F₂(|u|²,|v|²)|v|² - F(|u|²,|v|²) ≥ aF(|u|²,|v|²) ≥ 0, a > 0.