[1] Alexander Bogomolny, The Butterfly Theorem.
[2] Bismarck et al, MathLinks topic #38178
("Blaikie theorem (for all friends of involutions)").
[3] Mathservant et al, MathLinks topic #69971
("Quadrilateral [quadrilateral bisects segment on a
parallel]").
[4] Liyi et al, MathLinks topic #5326 ("a
quadrangle inscribed in a circle").
[5] Vladimir, Quadrilateral inversions.
[6] Pascual2005 et al, MathLinks topic #27608
("Nice circles intersection").
[7] Grobber et al, MathLinks topic #4919 ("diameter
AB [prove MK perpendicular to OK]").
[8] Jhaussmann5 et al, MathLinks topic #18801
("cyclic quadrilateral and diagonals: OY perp. XY").
[9] Juancarlos et al, MathLinks topic #15668
("Miquel point of cyclic quadrilateral").
[10] Juancarlos et al, MathLinks topic #15133
("Cyclic quadrilateral property: R^2 = OP * OQ").
[11] Max1546 et al, MathLinks topic #110196
("Proving an angle").
[12] Elchuy et al, MathLinks topic #110996 ("Equal
segments?").
[13] EDSON_ et al, MathLinks topic #108346 ("Tangent
to circumcircle at antipode meets sideline").
[14] Fuzzylogic et al, MathLinks topic #13061 ("A
Variation of The Butterfly Problem").
[15] Jhaussmann5 et al, MathLinks topic #18781
("cyclic quadrilateral and line [extended butterfly
theorem]").
[16] Virgil Nicula, MathLinks topic #73560 ("About
the butterfly theorem.").
On cyclic quadrilaterals and the butterfly theorem
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Darij Grinberg