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Dynamical systems on measure spaces, invariant measures, ergodic averages, mixing properties.

1 vote
0 answers
65 views

"Saddle connection" on a translation surface

A saddle connection on a translation surface $\omega$ is a geodesic in the flat metric determined by $\omega$ joining two zeros with no zeros in its interior. Athreya, Jayadev S., and Howard Masur. …
Joseph O'Rourke's user avatar
5 votes
0 answers
163 views

Pocket billiards with balls in general position

There were at least two earlier MO questions about ideal pocket billiards. (Ideal: frictionless, perfectly elastic collisions.) Perfectly centered break of a perfectly aligned pool ball rack. Does t …
Joseph O'Rourke's user avatar
18 votes
0 answers
480 views

Trapping lightrays with segment mirrors

Q. Is it possible to trap all the light from one point source by a finite collection of two-sided disjoint segment mirrors? I posed this question in several forums before (e.g., here and in an ea …
Joseph O'Rourke's user avatar
15 votes
2 answers
2k views

Are rounded rectangle billiard dynamics ergodic?

Bunimovich proved that the billiard-ball dynamics in the Bunimovich stadium is ergodic.             (Image from Microwave_billiards_and_quantum_chaos.) Q. Is it known that the billiard-ball dynami …
Joseph O'Rourke's user avatar
28 votes
0 answers
822 views

Blocking light with mirrored convex objects

There is a long-unsolved problem posed by Janos Pach, sometimes known as the enchanted forest problem, which asks if it is possible to block a point light source in the plane from reaching infinity by …
Joseph O'Rourke's user avatar
24 votes
2 answers
1k views

Billiard dynamics for multiple balls

I am interested to learn to what extent results on billiards in polygons have been extended to multiple balls. Assume the balls have equal radii and the same mass, the same initial speed, and all coll …
Joseph O'Rourke's user avatar
7 votes
2 answers
736 views

How quickly will billiard trajectories cluster?

Suppose you launch $n$ point-particles on distinct reflecting nonperiodic billiard trajectories inside a convex polygon. Assume they all have the same speed. Define an $\epsilon$-cluster as a configur …
Joseph O'Rourke's user avatar
33 votes
4 answers
3k views

Does there exist a shot in ideal pocket billiards?

Assume you have one shot with the cue ball in pocket billiards (a.k.a. pool), with the game idealized in that no spin is placed on the cue ball in the initial shot, all collisions between billiard bal …
Joseph O'Rourke's user avatar
16 votes
3 answers
2k views

A random walk on random lines

I am wondering if this random walk remains finite with positive probability. Start with three lines $A,B,C$ that are extensions of an equilateral triangle. Let $p_0$ be one corner. Generate a line $L_ …
Joseph O'Rourke's user avatar
17 votes
2 answers
2k views

Random walk is to diffusion as self-avoiding random walk is to ...?

One can view a random walk as a discrete process whose continuous analog is diffusion. For example, discretizing the heat diffusion equation (in both time and space) leads to random walks. Is there a …
Joseph O'Rourke's user avatar
20 votes
5 answers
1k views

Iterated Circumcircle

Take three noncollinear points (a,b,c), compute the center of their circumcircle x, and replace a random one of a,b,c with x. Repeat. It seems this process may converge to a point, assuming no collin …
Joseph O'Rourke's user avatar