The claim that performing an orbital inclination change at apoapsis maximizes fuel efficiency is a fundamental principle of orbital mechanics derived from the Oberth effect and vis-viva equations, requiring no specific citation.
The claim is a well-established fundamental principle of orbital mechanics (specifically, changing inclination where velocity is lowest, i.e., apoapsis, minimizes the velocity vector change magnitude and thus fuel consumption). None of the retrieved papers directly address or evaluate this standard textbook fact as their primary research topic, making it common knowledge in aerospace engineering.