I don't have a citation at hand, but the proof is simple enough. It does rely on the triangle inequality and can be used to construct a quick approximator for t

I don't have a citation at hand, but the proof is simple enough. It does rely on the triangle inequality and can be used to construct a quick approximator for this Euclidean variation of TSP. Granted, the math is different on a surface of a sphere. The error could be caused by imprecise input values.