Rendezvous problem

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The rendezvous dilemma is a logical dilemma, typically formulated in this way:


Two people have a date in a park they have never been to before. Arriving separately in the park, they are both surprised to discover that it is a huge area and consequently they cannot find one another. In this situation each person has to choose between waiting in a fixed place in the hope that the other will find them, or else starting to look for the other in the hope that they have chosen to wait somewhere.

If they both choose to wait, they will never meet. If they both choose to walk there are chances that they meet and chances that they do not. If one chooses to wait and the other chooses to walk, then there is a theoretical certainty that they will meet eventually; in practice, though, it may take too long for it to be guaranteed. The question posed, then, is: what strategies should they choose to maximize their probability of meeting?

Examples of this class of problems are known as rendezvous problems. These problems were first introduced informally by Steve Alpern in 1976, [1] and he formalised the continuous version of the problem in 1995. [2] This has led to much recent research in rendezvous search. [3] Even the symmetric rendezvous problem played in n discrete locations (sometimes called the Mozart Cafe Rendezvous Problem) [4] has turned out to be very difficult to solve, and in 1990 Richard Weber and Eddie Anderson conjectured the optimal strategy. [5] Only recently has the conjecture been proved for n = 3 by Richard Weber. [6] This was the first non-trivial symmetric rendezvous search problem to be fully solved. Note that the corresponding asymmetric rendezvous problem has a simple optimal solution: one player waits at his original location and the other player looks for him using a random permutation of the locations.

Steve Alpern is a professor of Operational Research at the University of Warwick, where he recently moved after working for many years at the London School of Economics. His early work was mainly in the area of dynamical systems and ergodic theory, but his more recent research has been concentrated in the fields of search games and rendezvous. He informally introduced the rendezvous problem as early as 1976. His collaborators include Shmuel Gal, Vic Baston and Robbert Fokkink.

Richard Robert Weber is a mathematician working in operational research. He is Churchill Professor of Mathematics for Operational Research in the Statistical Laboratory, University of Cambridge.

As well as being problems of theoretical interest, rendezvous problems include real-world problems with applications in the fields of synchronization, operating system design, operations research, and even search and rescue operations planning.

Synchronization coordination of events to operate a system in unison

Synchronization is the coordination of events to operate a system in unison. The conductor of an orchestra keeps the orchestra synchronized or in time. Systems that operate with all parts in synchrony are said to be synchronous or in sync—and those that are not are asynchronous.

Operating system collection of software that manages computer hardware resources

An operating system (OS) is system software that manages computer hardware and software resources and provides common services for computer programs.

Operations research, or operational research (OR) in British usage, is a discipline that deals with the application of advanced analytical methods to help make better decisions. Further, the term operational analysis is used in the British military as an intrinsic part of capability development, management and assurance. In particular, operational analysis forms part of the Combined Operational Effectiveness and Investment Appraisals, which support British defense capability acquisition decision-making.

Deterministic rendezvous problem

The deterministic rendezvous problem is a variant of the rendezvous problem where the players, or robots, must find each other by following a deterministic sequence of instructions. Although each robot follows the same instruction sequence, a unique label assigned to each robot is used for symmetry breaking. [7]

Symmetry breaking Physical process transitioning a system from a symmetric state to a more ordered state

In physics, symmetry breaking is a phenomenon in which (infinitesimally) small fluctuations acting on a system crossing a critical point decide the system's fate, by determining which branch of a bifurcation is taken. To an outside observer unaware of the fluctuations, the choice will appear arbitrary. This process is called symmetry "breaking", because such transitions usually bring the system from a symmetric but disorderly state into one or more definite states. Symmetry breaking is thought to play a major role in pattern formation.

See also

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In computer science, the sleeping barber problem is a classic inter-process communication and synchronization problem between multiple operating system processes. The problem is analogous to that of keeping a barber working when there are customers, resting when there are none, and doing so in an orderly manner.

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  1. Alpern, Steve (1976), Hide and Seek Games, Seminar, Institut fur Hohere Studien, Wien, 26 July.
  2. Alpern, Steve (1995), "The rendezvous search problem", SIAM Journal on Control and Optimization, 33 (3): 673–683, doi:10.1137/S0363012993249195, MR   1327232
  3. Alpern, Steve; Gal, Shmuel (2003), The Theory of Search Games and Rendezvous, International Series in Operations Research & Management Science, 55, Boston, MA: Kluwer Academic Publishers, ISBN   0-7923-7468-1, MR   2005053 .
  4. Alpern, Steve (2011), "Rendezvous search games", in Cochran, James J., Wiley Encyclopedia of Operations Research and Management Science, Wiley, doi:10.1002/9780470400531.eorms0720 .
  5. Anderson, E. J.; Weber, R. R. (1990), "The rendezvous problem on discrete locations", Journal of Applied Probability, 27 (4): 839–851, doi:10.2307/3214827, MR   1077533 .
  6. Weber, Richard (2012), "Optimal symmetric Rendezvous search on three locations" (PDF), Mathematics of Operations Research, 37 (1): 111–122, doi:10.1287/moor.1110.0528, MR   2891149 .
  7. Ta-Shma, Amnon; Zwick, Uri (April 2014). "Deterministic rendezvous, treasure hunts, and strongly universal exploration sequences". ACM Transactions on Algorithms. 10 (3). 12.