Paris Pamfilos, Orthocycles, bicentrics, and orthodiagonals,
Forum Geometricorum, 7 (2007) 73--86.

Abstract:  We study configurations involving a circle (orthocycle) intimately related to a cyclic quadrilateral. As an illustration of the usefulness of this circle we explore its connexions with bicentric (bicentrics) and orthodiagonal quadrilaterals (orthodiagonals) reviewing the more or less known facts and revealing some other properties of these classes of quadrilaterals.

[ps file][pdf]