Space-like form of Bethe-Salpeter equation: Bound state

According to Dirac's principle, we apply the space-like consistency conditions in a relativistic theory to two-particle system and then define the space-like wavefunctions through introducing a space-like factor, which is equivalent to Bethe-Salpeter wavefunction in physical content. The space-like form of Bethe-Salpeter equation of bound states is derived in terms of the universal rearranging technology of interaction kernel. Its advantages are of explicit Lorentz-covariant form and the difficulty of ghost states is automatically overcome. We also discuss the normalization condition of the space-like function.