ETH Zurich – Distributed Computing – Klaus-Tycho Förster, Rijad Nuridini, Jara Uitto, Roger Wattenhofer Lower Bounds for the Capture Time: Linear, Quadratic, and Beyond
How to catch a robber on a graph? The game of Cops and Robbers
The rules of the game
The Cop is placed first C
The Robber may then choose a placement CR
Next, they alternate in moves CR
CR
CR
CR
The Cop won! C
C
How many moves does the cop need?
Catch multiple? RRRC
Upper bound to catch ℓ robbers
Lower bound to catch ℓ robbers … …
C … …
CR R R … …
CR R R … …
CR R R … …
R CR R … …
R CR R … …
CR R R … …
CR R R … …
R CR R … …
R C R … …
R C … …
R C … …
CR R … …
Summary so far
What about multiple cops and one robber?
Let’s start with two cops and one robber …
R C C … one cop not enough
Let’s start with two cops and one robber C R C …
Beyond two cops? … ??
Multiple cops and one robber …
Summary so far
What about multiple cops and multiple robbers?
Are we done? Multiple cops and multiple robbers … … … …
… … … …
… … … … C Prevents robbers from crossing
C ……
C ……
C
C
C
Construction of the ring … … …… …
Robber placement Robbers choose side with less cops R R R
Robber strategy R R R
R R R
R R R C Can catch half of the robbers!
Robber strategy R R R C
R R R C
R R R C
R R R C
Summary
ETH Zurich – Distributed Computing – Klaus-Tycho Förster, Rijad Nuridini, Jara Uitto, Roger Wattenhofer Lower Bounds for the Capture Time: Linear, Quadratic, and Beyond