Two Butterflies Theorem
What is it about?
A Mathematical Droodle

Let $P$ be a point not on a circle $C.$ Let $f_{P}$ be the transformation taking each point $A$ on $C$ to the other intersection of $AP$ with $C.$ Then there is a mobius transform $g_{P}$ such that $f_{P}$ is the restriction of $g_{P}$ to $C.$

Proof

If $P$ lies outside $C,$ then by the intersecting chords theorem we may take $g_{P}$ to be inversion in a circle of suitable radius centred at $P.$ If $P$ lies inside $C,$ it is merely necessary to add an extra half-twist centred at $P.$

Now let $g$ be the mobius transformation obtained by composing $g_{P},$ $g_{Q},$ $g_{R}$ and $g_{S}.$ Then $g$ has three fixed points on $C:$ $A,$ $B$ and $K.$ So $g$ is the identity on $C$ and so $g(K') = K'.$ The result follows.

Note that the same argument will give the following interesting porism as a generalisation:

Let $A$ and $B$ lie on a circle $S,$ and let $P_{1}, \ldots, P_{2n}$ be points on $AB$ distinct from $A$ and $B.$ If there is any point $K$ on $S,$ other than $A$ or $B,$ with the property that there are points $K_{1}, \ldots, K_{2n+1}$ on $S$ with $P_{i}$ lying on $K_{i} K_{i+1}$ for all $i\le 2n,$ and $K_{1} = K_{2n+1} = K,$ then all points on $S$ have this property.

Remark

We may generalise still further. The key point to note is that in the lemma we may in fact construct $g_{P}$ knowing only $P,$ $A$ and $B$ where $P,$ $A$ and $B$ are collinear and $A$ and $B$ lie on the circle $C.$ Then, clarifying the proof by noting that $g$ preserves the orientation of the plane and so is not the identity merely on $C$ but everywhere, we may observe that it is not necessary for the circles $KLMN$ and $K'L'M'N'$ to be the same; we need only require that both pass through both $A$ and $B.$

To be less formal, in the two butterflies theorem the two butterflies may live on different circles, provided that those circles meet at $A$ and $B.$

As a further generalization, it comes out that the two circles need not intersect: the theorem remains valid as long as $A$ and $B$ lie on the radical axis of the two circles.

Finally, like, say, Pascal's theorem, the theorem discussed here, deals only with incidence of points and lines and, therefore, is true for any non-degenerate conic. We have a demonstration of that statement for ellipses along with a demonstration of porism suggested by Nathan Bowler.

We have an additional proof of the main result. The proof is by L. Hoehn of a statement first proved by D. Jones.

With a reference to the above diagram, Jones' result appears as

Let $PQ$ be a fixed chord of a circle and let "butterfly $R$" and "butterfly $S$" be inscribed in the circle and oriented such that their wings cut $PQ$ (in order from left to right) at $R_{4},$ $R_{3},$ $R_{2},$ $R_{1}$ and $S_{1},$ $S_{2},$ $S_{3},$ $S_{4},$ respectively. If $PR_{1} = QS_{1},$ $PR_{2} = QS_{2}$ and $PR_{3} = QS_{3},$ then $PR_{4} = QS_{4}.$

(A reflection in the perpendicular bisector of $PQ$ immediately shows that the "Double Butterfly Theorem" is equivalent to the "Two Butterflies Theorem".)

We apply Hiroshi Haruki's Lemma twice.

By applying Haruki's lemma twice to points $A$ and $B$ and fixed chords $PQ$ and $CD$ of butterfly $R,$ we have

$m\cdot (b + c + e) / (a - m) = (a + b)\cdot e / c,$

sine both are equal to the same constant. Similarly,

$n\cdot (b + c + e) / (a - n) = (a + b)\cdot e / c,$

for butterfly $S.$ By transitivity of equality, we easily get $m = n,$ which completes the proof.

References

1. D. Jones, A Double Butterfly Theorem, Math Magazine 49 (1976), pp. 86-87
2. L. Hoehn, A New Proof of the Double Butterfly Theorem, Math Magazine 63 (1990), pp. 256-257
3. V. V. Prasolov, Essays On Numbers And Figures, AMS, 2000

Butterfly Theorem and Variants

1. Butterfly theorem
2. 2N-Wing Butterfly Theorem
3. Better Butterfly Theorem
4. Butterflies in Ellipse
5. Butterflies in Hyperbola
6. Butterflies in Quadrilaterals and Elsewhere
7. Pinning Butterfly on Radical Axes
8. Shearing Butterflies in Quadrilaterals
9. The Plain Butterfly Theorem
10. Two Butterflies Theorem
11. Two Butterflies Theorem II
12. Two Butterflies Theorem III
13. Algebraic proof of the theorem of butterflies in quadrilaterals
14. William Wallace's Proof of the Butterfly Theorem
15. Butterfly theorem, a Projective Proof
16. Areal Butterflies
17. Butterflies in Similar Co-axial Conics
18. Butterfly Trigonometry
19. Butterfly in Kite
20. Butterfly with Menelaus
21. William Wallace's 1803 Statement of the Butterfly Theorem
22. Butterfly in Inscriptible Quadrilateral
23. Camouflaged Butterfly
24. General Butterfly in Pictures
25. Butterfly via Ceva
26. Butterfly via the Scale Factor of the Wings
27. Butterfly by Midline
28. Stathis Koutras' Butterfly
29. The Lepidoptera of the Circles
30. The Lepidoptera of the Quadrilateral
31. The Lepidoptera of the Quadrilateral II
32. The Lepidoptera of the Triangle
33. Two Butterflies Theorem as a Porism of Cyclic Quadrilaterals
34. Two Butterfly Theorems by Sidney Kung
35. Butterfly in Complex Numbers

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