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Empty pseudo-triangles in point sets
Ahn, H.K.,Bae, S.W.,van Kreveld, M.,Reinbacher, I.,Speckmann, B. North Holland ; Elsevier Science Ltd 2011 Discrete applied mathematics Vol.159 No.18
We study empty pseudo-triangles in a set P of n points in the plane, where an empty pseudo-triangle has its vertices at the points of P, and no points of P lie inside. We give bounds on the minimum and maximum number of empty pseudo-triangles. If P lies inside a triangle whose corners must be the convex vertices of the pseudo-triangle, then there can be between Θ(n<SUP>2</SUP>) and Θ(n<SUP>3</SUP>) empty pseudo-triangles. If the convex vertices of the pseudo-triangle are also chosen from P, this number lies between Θ(n<SUP>3</SUP>) and Θ(n<SUP>6</SUP>). If we count only star-shaped pseudo-triangles, the bounds are Θ(n<SUP>2</SUP>) and Θ(n<SUP>5</SUP>). We also study optimization problems: minimizing or maximizing the perimeter or the area over all empty pseudo-triangles defined by P. If P lies inside a triangle whose corners must be used, we can solve these problems in O(n<SUP>3</SUP>) time. In the general case, the running times are O(n<SUP>6</SUP>) for the maximization problems and O(nlogn) for the minimization problems.