This is commonly taken as a general solution to the free Klein–Gordon equation. Note that because the initial Fourier transformation contained Lorentz invariant quantities like p ⋅ ⋅ x = p μ μ x μ μ only, the last expression is also a Lorentz invariant solution to the Klein–Gordon equation. If one does not require Lorentz invariance, one can absorb the 1 / 2 E ( p ) -factor into the coefficients A ( p ) and B ( p ) .