Naive
Repeatedly run convex hull. O(N² log N) worst case if many layers.
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Chazelle O(N log N)
Uses hull tree data structure. All layers in single log factor overhead.
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Layer number
Point in layer k = removed after k-1 hull removals. Onion depth = max layer number.
Applications
Robust statistics (peel outliers). Ham-sandwich theorem. Facility location.