Poncelet's closure theorem
In geometry, Poncelet's porism (sometimes referred to as Poncelet's closure theorem), named after French engineer and mathematician Jean-Victor Poncelet, states the following: Let C and D be two plane conics. If it is possible to find, for a given n > 2, one n-sided polygon that is simultaneously inscribed in C and circumscribed around D (i.e., a bicentric polygon), then it is possible to find infinitely many of them.
Sketch of proof
View C and D as projective curves in P2. For simplicity, assume that C and D meet transversely. Then by Bézout's theorem, C ∩ D consists of 4 (complex) points. For d in D, let ℓd be the tangent line to D at d. Let X be the subvariety of C × D consisting of (c,d) such that ℓd passes through c. Given c, the number of d with (c,d) ∈ X is 1 if c ∈ C ∩ D and 2 otherwise. Thus the projection X → C ≃ P1 presents X as a degree 2 cover ramified above 4 points, so X is an elliptic curve (once we fix a base point on X). Let be the involution of X sending a general (c,d) to the other point (c,d′) with the same first coordinate. Any involution of an elliptic curve with a fixed point, when expressed in the group law, has the form x → p − x for some p, so has this form. Similarly, the projection X → D is a degree 2 morphism ramified over the contact points on D of the four lines tangent to both C and D, and the corresponding involution has the form x → q − x for some q. Thus the composition is a translation on X. If a power of has a fixed point, that power must be the identity. Translated back into the language of C and D, this means that if one point c ∈ C (equipped with a corresponding d) gives rise to an orbit that closes up (i.e., gives an n-gon), then so does every point. The degenerate cases in which C and D are not transverse follow from a limit argument.
- Bos, H. J. M.; Kers, C.; Oort, F.; Raven, D. W. Poncelet's closure theorem. Expositiones Mathematicae 5 (1987), no. 4, 289–364.
- David Speyer on Poncelet's Porism
- D. Fuchs, S. Tabachnikov, Mathematical Omnibus: Thirty Lectures on Classic Mathematics
- Java applet by Michael Borcherds showing the cases n = 3, 4, 5, 6, 7, 8 (including the convex cases for n = 7, 8) made using GeoGebra.
- Java applet by Michael Borcherds showing Poncelet's Porism for a general Ellipse and a Parabola made using GeoGebra.
- Java applet by Michael Borcherds showing Poncelet's Porism for 2 general ellipses (order 3) made using GeoGebra.
- Java applet by Michael Borcherds showing Poncelet's Porism for 2 general ellipses (order 5) made using GeoGebra.
- Java applet by Michael Borcherds showing Poncelet's Porism for 2 general ellipses (order 6) made using GeoGebra.
- Java applet showing the exterior case for n = 3 at National Tsing Hua University.
- Article on Poncelet's Porism at Mathworld.