Elliptic-billiard invariant k_{701}

Definitions and mathematical statement

Shared notation.

$\mathbb N=\{1,2,\ldots\}$, and $\mathbb Z,\mathbb Q,\mathbb R,\mathbb C$ denote the integers, rationals, reals and complex numbers. Any other conventions are specified locally.

Fix the ellipse $E:x^2/a^2+y^2/b^2=1$, where $a>b>0$, with center $O=(0,0)$ and foci $f_1,f_2=(\pm\sqrt{a^2-b^2},0)$. Fix a confocal elliptic caustic inside $E$ admitting a Poncelet family of $N$-periodic billiard polygons $P=(P_1,\ldots,P_N)$, with $N\geq3$. Their vertices lie on $E$, their sides are tangent to the caustic, and the incoming and outgoing sides obey the billiard reflection law. Indices are cyclic. The outer polygon $P'$ has vertices given by intersections of consecutive tangent lines to $E$ at the $P_i$; the inner polygon $P''$ has vertices at the contacts of the sides of $P$ with the caustic. All constancy assertions concern variation within this fixed family. For any ordered polygon $R=(R_i)_{i=1}^N$, use the signed area \[S(R)=\frac12\sum_{i=1}^N R_i\times R_{i+1},\qquad (x,y)\times(u,v)=xv-yu.\] Write $A=S(P)$, $A'=S(P')$ and $A''=S(P'')$. A pedal polygon $\operatorname{ped}_M(R)$ has vertices the perpendicular feet from $M$ to the lines containing the sides of $R$. An antipedal polygon $\operatorname{ant}_M(R)$ has vertices at consecutive intersections of the lines through $R_i$ perpendicular to $R_i-M$. Define \[A_M=S(\operatorname{ped}_M(P)),\quad A'_M=S(\operatorname{ped}_M(P')),\quad A_M^*=S(\operatorname{ant}_M(P)).\] Primes or double primes on analogous quantities refer to $P'$ or $P''$. For $R$, its vertex centroid and signed area centroid are \[C_0(R)=\frac1N\sum_i R_i,\qquad C_2(R)=\frac{1}{6S(R)}\sum_i(R_i\times R_{i+1})(R_i+R_{i+1}).\] Constructions and quotients are understood wherever their defining intersections and denominators exist. For an ordered polygon $R$, its evolute polygon $R_{\mathrm{ev}}$ has vertices at intersections of successive perpendicular bisectors of the sides of $R$. Put $A_{\mathrm{ev}}=S(P_{\mathrm{ev}})$, $A'_{\mathrm{ev}}=S(P'_{\mathrm{ev}})$ and $A''_{\mathrm{ev}}=S(P''_{\mathrm{ev}})$.

Statement recorded in the supplied source.

Prove that for every $N>4$, $A/A_{\mathrm{ev}}$ is constant over the family (invariant $k_{701}$). [S2]

Short English statement

For periods greater than four, prove that the signed area of the billiard polygon divided by that of its evolute polygon stays constant.

Sources

• S1 ULAM.AI and the UnsolvedMath Contributors, UnsolvedMath: A Curated Collection of Open Mathematics Problems, record 5100039 (AMR-050-0039). CC BY 4.0. https://huggingface.co/datasets/ulamai/UnsolvedMath.
• S2 Dan Reznik, Ronaldo Garcia and Jair Koiller, Eighty New Invariants in the Elliptic Billiard, arXiv:2004.12497v11 (29 October 2020), §3.8 and Table 8, pp. 8--9. https://arxiv.org/abs/2004.12497v11.