 Solution

Take the same $e^{-i\omega t}$ convention and an outgoing field. Define the scattering potential with the positive-contrast sign,The outgoing Green function for the three-dimensional Helmholtz equation isThus the Lippmann-Schwinger equation has a plus sign:Replacing the total field inside the integral by the incident field gives the Born approximation for scalar wave scatteringMore generally, iterating the integral equation gives the Born series ${\psi }_s={\sum }_{j\ge 1}{T}^j{\psi }_i$; its first term is the boxed expression. A sufficient convergence condition is $\Vert T\Vert <1$ on the chosen space of fields in $V$, and accurate single-scattering truncation needs the neglected terms to be small. Weak refractive contrast is useful, but for a sufficiently large or resonant scatterer it is not by itself a guarantee of negligible multiple scattering.