Generalized eigenfunctions of relativistic Schroedinger operators I

Generalized eigenfunctions of the 3-dimensional relativistic Schr\"odinger operator $\sqrt{\Delta} + V(x)$ with $|V(x)|\le C < x >^{{-\sigma}}$, $\sigma > 1$, are considered. We show that the generalized eigenfunctions can be expressed as the sum of plane waves and solutions to the time-independent relativistic Schr\"odinger equation with the radiation condition. If $\sigma >3$, then we can give pointwise estimates of the differences between the sums and the solutions.