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The '''pebble motion problems''', or '''pebble motion on graphs''', are a set of related problems in [[graph theory]] dealing with the movement of multiple objects ("pebbles") from vertex to vertex in a [[Graph (mathematics)|graph]] with a constraint on the number of pebbles that can occupy a vertex at any time. Pebble motion problems occur in domains such as multi-[[robot]] [[motion planning]] (in which the pebbles are robots) and [[network routing]] (in which the pebbles are [[Data packet|packets]] of data). The best-known example of a pebble motion problem is the famous [[15 puzzle]] where a disordered group of fifteen tiles must be rearranged within a 4x4 grid by sliding one tile at a time.


==Theoretical formulation==
The general form of the pebble motion problem is Pebble Motion on Graphs<ref name="Kornhauser">[http://www2.computer.org/portal/web/csdl/doi/10.1109/SFCS.1984.715921 ]{{dead link|date=September 2013}}</ref> formulated as follows:


Let <math>G = (V,E)</math> be a graph with <math>n</math> vertices. Let <math>P = \{1,\ldots,k\}</math> be a set of pebbles with <math>k < n</math>. An arrangement of pebbles is a mapping <math>S : P \rightarrow V</math> such that <math>S(i) \neq S(j)</math> for <math>i \neq j</math>. A move <math>m = (p, u, v)</math> consists of transferring pebble <math>p</math> from vertex <math>u</math> to adjacent unoccupied vertex <math>v</math>. The Pebble Motion on Graphs problem is to decide, given two arrangements <math>S_0</math> and <math>S_+</math>, whether there is a sequence of moves that transforms <math>S_0</math> into <math>S_+</math>.
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===Variations===
Common variations on the problem limit the structure of the graph to be:
* a [[Tree graph|tree]]<ref name="Auletta">{{cite web|url=http://www.springerlink.com/content/fnq2nmmd7g7dpu3r/ |title=A Linear-Time Algorithm for the Feasibility of Pebble Motion on Trees - Springer |publisher=Springerlink.com |date= |accessdate=2013-09-27}}</ref>
* a [[Lattice graph|square grid]],<ref name="Calinescu">{{cite web|url=http://www.springerlink.com/content/f2v1985q85261410/ |title=Reconfigurations in Graphs and Grids - Springer |publisher=Springerlink.com |date= |accessdate=2013-09-27}}</ref>
* a [[Biconnected graph|bi-connected]]<ref name="Surynek">{{cite web|author=First Name Middle Name Last Name |url=http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5152326 |title=IEEE Xplore - A novel approach to path planning for multiple robots in bi-connected graphs |doi=10.1109/ROBOT.2009.5152326 |publisher=Ieeexplore.ieee.org |date=2009-05-17 |accessdate=2013-09-27}}</ref> graph.
 
Another set of variations consider the case in which some<ref name="Papadimitriou">[http://www2.computer.org/portal/web/csdl/doi/10.1109/SFCS.1994.365740 ]{{dead link|date=September 2013}}</ref> or all<ref name="Calinescu" /> of the pebbles are unlabeled and interchangeable.
 
Other versions of the problem seek not only to prove reachability but to find a (potentially optimal) sequence of moves (i.e. a plan) which performs the transformation.
 
==Complexity==
Finding the shortest path in the pebble motion on graphs problem (with labeled pebbles) is known to be [[NP hard|NP-hard]]<ref name="Ratner">{{cite web|url=http://portal.acm.org/citation.cfm?id=102409 |title=The (n2-1)-puzzle and related relocation problems |doi=10.1016/S0747-7171(08)80001-6 |publisher=Portal.acm.org |date= |accessdate=2013-09-27}}</ref> and [[APX|APX-hard]].<ref name="Calinescu" />  The unlabeled problem can be solved in polynomial time when using the cost metric mentioned above (minimizing the total number of moves to adjacent vertices), but is [[NP hard|NP-hard]] for other natural cost metrics.<ref name="Calinescu" />
 
== References ==
<!--- See [[Wikipedia:Footnotes]] on how to create references using <ref></ref> tags which will then appear here automatically -->
{{Reflist}}
 
{{DEFAULTSORT:Pebble Motion Problems}}
[[Category:Multi-agent systems]]
[[Category:Automated planning and scheduling]]
[[Category:Computational problems in graph theory]]

Latest revision as of 20:43, 21 March 2014


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