Martin Josefsson, Characterizations of orthodiagonal quadrilaterals,
Forum Geometricorum, 12 (2012) 13--25.

Abstract.   We prove ten necessary and sufficient conditions for a convex quadrilateral to have perpendicular diagonals. One of these is a quite new eight point circle theorem and three of them are metric conditions concerning the nonoverlapping triangles formed by the diagonals.

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