Evacuation
Robots start at the centre of a disk (or a polygon) and must all leave through an exit at an unknown point of the boundary, as fast as possible.
Problem statement and measuresexpand
\(k\) unit-speed robots start together at the centre of a unit disk (or at the centroid, or at a given point of the boundary, of a unit equilateral triangle or square). An exit lies at an unknown point of the boundary; a robot recognises it only on reaching it. Time is the worst case, over exit positions, of the time until the last robot reaches the exit (in priority evacuation, where one robot is the queen: until the queen does; in weighted evacuation of two robots, a weighted average of their arrival times; in search, until a non-faulty robot has reached the exit and every non-faulty robot knows where it is).
Measures: Evacuation time, worst-case time until the last robot (or the queen) reaches the exit, or the weighted average or search time where the goal says so; unit disk, unit speed.
Line, priority evacuation, queen and one face-to-face servant
verifiedFace-to-face, the queen reaches the exit with ratio at most \(2+\sqrt5 \approx 4.236\), far below \(9\); no lower bound is known.
Assumed (added on the last edge are outlined)
Not assumed
Formal definitions of this setting
- Domain: line
- The robots start at the origin of the real line; the exit is at an unknown point at distance at least \(1\); the measure is the competitive ratio, the evacuation time divided by that distance.
- Number of robots: two robots
- Two robots.
- Goal: priority (queen)
- One designated robot (the queen) must reach the exit; the others (servants) only help; the task ends when the queen exits.
- Exits: one exit
- One exit, at an unknown point of the boundary.
- Communication: face-to-face
- Face-to-face: two robots exchange information only when they are at the same point at the same time.
- Faults: no faults
- Every robot detects and reports the target.
- Speeds: equal speeds
- All robots move at most at unit speed.
- Randomness: deterministic
- Trajectories are fixed functions of the information received.
What is known
Results, in order of publication
Introductory and current best (Theorem 6.2): the queen walks one way at unit speed, the servant the other way at speed \(\sqrt5-2\); a servant that finds the exit catches the queen and brings her back. If the servant must also reach the exit, \(6.60258\) (Theorem 6.1).
Open questions
A lower bound for priority evacuation of a queen and one face-to-face servant on the line.
Notes
Checked against the full text of Georgiou & Lucier 2023.
What carries over from neighbouring settings
Read off the order of assumptions, not checked case by case: results may carry extra assumptions (shown on their pages), and asynchronous time is measured in epochs.
Algorithms from weaker settings work here
Lower bounds from stronger settings hold here
Know a better result for this setting, or found a mistake? Suggest a result · Report an error. Accepted contributions are acknowledged.