A note on a system of cubic nonlinear Klein-Gordon equations in one space dimension

We study the Cauchy problem for a system of cubic nonlinear Klein-Gordon equations in one space dimension. Under a suitable structural condition on the nonlinearity, we will show that the solution exists globally and decays of the order $O(t^{-1/2})$ in $L^\infty$ as $t$ tends to infinity without the condition of a compact support on the Cauchy data which was assumed in the previous works.