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.
[ps file] [pdf]