Daniela Ferrarello, Maria Flavia Mammana and Mario Pennisi,
Pedal polygons,
Forum Geometricorum, 13 (2013) 153--164.

Abstract. We study the pedal polygon H1H2...Hn of a point P with respect to a polygon P, where the points Hi are the feet of the perpendiculars drawn from P to the sides of P. In particular we prove that if P is a quadrilateral which is not a parallelogram, there exists one and only one point P for which the points Hi are collinear.

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