The scattering problem for the two-dimensional nonlinear Klein–Gordon equation with exponential nonlinearity 𝒩(u) is studied. It has been shown that the scattering operator 𝑺 ∶ (f−, g−) ↦ (f+, g+) is well defined on a neighborhood in the energy space ℋ ∶ = H1(ℝ2) ⊕ L2(ℝ2) of 0. In this paper, we establish inequalities for the smoothness and decay of the output data (f+, g+) dependent on the smoothness and decay of the input data (f−, g−) and the smoothness of 𝒩. The inequalities imply that (f+, g+) belongs to the Schwartz space 𝒮(ℝ2) ⊕ 𝒮(ℝ2) if (f−,g−) is in 𝒮(ℝ2) ⊕ 𝒮(ℝ2) and small in the sense of ℋ, and that we have a simple sufficient condition for (f−, g−) and 𝒩such that all order partial derivatives of f+ and g+ decay more rapidly than a similar exponential function.
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