Interactions can be included by adding an interaction Hamiltonian. For a "φ"4 theory, this corresponds to adding a Wick ordered term "g":"φ"4:/4! to the Hamiltonian, and integrating over "x". Scattering amplitudes may be calculated from this Hamiltonian in the interaction picture. These are constructed in perturbation theory by means of the Dyson series, which gives the time-ordered products, or "n"-particle Green's functions formula_32 as described in the Dyson series article. The Green's functions may also be obtained from a generating function that is constructed as a solution to the Schwinger–Dyson equation.