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{{unsolved|mathematics|Are graphs uniquely determined by their subgraphs?}}
Informally, the '''reconstruction conjecture''' in [[graph theory]] says that graphs are determined uniquely by their subgraphs.  It is due to [[Paul Kelly (mathematician)|Kelly]]<ref name=Kelly57>Kelly, P. J., [http://projecteuclid.org/getRecord?id=euclid.pjm/1103043674 A congruence theorem for trees], ''Pacific J. Math.'' '''7''' (1957), 961&ndash;968.</ref> and [[Stanislaw Ulam|Ulam]].<ref name=Ulam60>Ulam, S. M., A collection of mathematical problems, Wiley, New York, 1960.</ref>
 
==Formal statements==
 
Given a graph <math>G = (V,E)</math>, a '''vertex-deleted subgraph''' of <math>G</math> is a [[Glossary of graph theory#Subgraphs|subgraph]] formed by deleting exactly one vertex from <math>G</math>. Clearly, it is an [[induced subgraph]] of <math>G</math>.
 
For a graph <math>G</math>, the '''deck of G''', denoted <math>D(G)</math>, is the [[multiset]] of all vertex-deleted subgraphs of <math>G</math>. Each graph in <math>D(G)</math> is called a '''card'''.  Two graphs that have the same deck are said to be '''hypomorphic'''.
 
With these definitions, the conjecture can be stated as:
 
* '''Reconstruction Conjecture:''' Any two hypomorphic graphs on at least three vertices are isomorphic.
 
(The requirement that the graphs have at least three vertices is necessary because both graphs on two vertices have the same decks.)
 
[[Frank Harary|Harary]]<ref name="Harary64">Harary, F., On the reconstruction of a graph from a collection of subgraphs. In ''Theory of Graphs and its Applications (Proc. Sympos. Smolenice, 1963)''. Publ. House Czechoslovak Acad. Sci., Prague, 1964, pp. 47–52.</ref> suggested a stronger version of the conjecture:
 
* '''Set Reconstruction Conjecture:''' Any two graphs on at least four vertices with the same sets of vertex-deleted subgraphs are isomorphic.
 
Given a graph <math>G = (V,E)</math>, an '''edge-deleted subgraph''' of <math>G</math> is a [[Glossary of graph theory#Subgraphs|subgraph]] formed by deleting exactly one edge from <math>G</math>.  
 
For a graph <math>G</math>, the '''edge-deck of G''', denoted <math>ED(G)</math>, is the [[multiset]] of all edge-deleted subgraphs of <math>G</math>Each graph in <math>ED(G)</math> is called an '''edge-card'''.
 
* '''Edge Reconstruction Conjecture:''' (Harary, 1964)<ref name="Harary64"/> Any two graphs with at least four edges and having the same edge-decks are isomorphic.
 
==Verification==
 
Both the reconstruction and set reconstruction conjectures have been verified for all graphs with at most 11 vertices ([[Brendan McKay|McKay]]<ref name=McKay97>McKay, B. D., Small graphs are reconstructible, ''Australas. J. Combin.'' '''15''' (1997), 123&ndash;126.</ref>).
 
In a probabilistic sense, it has been shown ([[Béla Bollobás|Bollobás]]<ref name=Bollobas90>Bollob&aacute;s, B., Almost every graph has reconstruction number three, ''J. Graph Theory'' '''14''' (1990), 1&ndash;4.</ref>) that almost all graphs are reconstructible.  This means that the probability that a randomly chosen graph on <math>n</math> vertices is not reconstructible goes to 0 as <math>n</math> goes to infinity.  In fact, it was shown that not only are almost all graphs reconstructible, but in fact that the entire deck is not necessary to reconstruct them &mdash; almost all graphs have the property that there exist three cards in their deck that uniquely determine the graph.
 
===Reconstructible graph families===
 
The conjecture has been verified for a number of infinite classes of graphs.
 
*[[Regular graph]]s<ref name=h74>{{Citation | last=Harary | first=F. | contribution=A survey of the reconstruction conjecture | series=Graphs and Combinatorics. [[Lecture Notes in Mathematics]]| pages=18–28 | year=1974 | publisher=Springer | doi=10.1007/BFb0066431 | title=A survey of the reconstruction conjecture | volume=406}}</ref>
*[[Tree (graph theory)|Trees]]<ref name=h74/>
*[[Connected graph|Disconnected graphs]]<ref name=h74/>
*[[Unit interval graph]]s <ref name=rim/>
*[[Separable graphs without end vertices]]
*[[Maximal planar graph]]s
*[[Maximal outerplanar graph]]s
*[[Outer planar graph]]s
*[[Critical blocks]]
 
==Recognizable properties==
{{unreferenced-section|date=August 2009}}
 
In context of the reconstruction conjecture, a [[graph property]] is called '''recognizable''' if one can determine the property from the deck of a graph. The following properties of graphs are recognizable:
 
*[[Degree sequence]]
*[[Tutte polynomial]]
*[[Planar graph|Planarity]]
*The types of [[spanning tree (mathematics)|spanning tree]]s in a graph
*[[Chromatic polynomial]]
*Being a [[perfect graph]] or an [[interval graph]], or some other subclasses of perfect graphs<!--to verify later which ones are listed there--> <ref name=rim>von Rimscha, M.: Reconstructibility and perfect graphs. ''Discrete Mathematics'' '''47''', 283–291 (1983)</ref>
 
==Reduction==
The reconstruction conjecture is true if all 2-connected graphs are reconstructible <ref name=yang>Yang Yongzhi:The reconstruction conjecture is true if all 2-connected graphs are reconstructible. ''Journal of graph theory'' '''12''', 237–243 (1988)</ref>
 
==Other structures==
 
It has been shown that the following are '''not''' in general reconstructible:
 
* [[graph (mathematics)#Directed_graph|Digraphs]]: Infinite families of non-reconstructible digraphs are known, including [[tournament (mathematics)|tournaments]] (Stockmeyer<ref name=Stockmeyer77>Stockmeyer, P. K., The falsity of the reconstruction conjecture for tournaments, ''J. Graph Theory'' '''1''' (1977), 19&ndash;25.</ref>) and non-tournaments (Stockmeyer<ref name=Stockmeyer81>Stockmeyer, P. K., A census of non-reconstructable digraphs, I: six related families, ''J. Combin. Theory Ser. B'' '''31''' (1981), 232&ndash;239.</ref>). A tournament is reconstructible if it is not strongly connected.<ref name=HararyPalmer>Harary, F. and Palmer, E., On the problem of reconstructing a tournament from sub-tournaments, ''Monatsh. Math.'' '''71''' (1967), 14&ndash;23.</ref> A weaker version of the reconstruction conjecture has been conjectured for digraphs, see [[New digraph reconstruction conjecture]].
* [[Hypergraph]]s ([[William Lawrence Kocay|Kocay]]<ref name=Kocay87>Kocay, W. L., A family of nonreconstructible hypergraphs, ''J. Combin. Theory Ser. B'' '''42''' (1987), 46&ndash;63.</ref>).
* [[Infinite graph]]s.  Let T be a tree on an infinite number of vertices such that every vertex has infinite degree.  The counterexample is T and 2T.  The question of reconstructibility for locally finite infinite graphs is still open.
 
==See also==
 
* [[New digraph reconstruction conjecture]]
 
==Further reading==
For further information on this topic, see the survey by [[Crispin St. J. A. Nash-Williams|Nash-Williams]].<ref name=NashWilliams78>[[Crispin St. J. A. Nash-Williams|Nash-Williams, C. St. J. A.]], The Reconstruction Problem, in ''Selected topics in graph theory'', 205&ndash;236 (1978).</ref>
 
==References==
 
<references/>
 
{{DEFAULTSORT:Reconstruction Conjecture}}
[[Category:Graph theory]]
[[Category:Conjectures]]

Revision as of 20:16, 27 January 2014

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Here is my homepage ... new launch ec Informally, the reconstruction conjecture in graph theory says that graphs are determined uniquely by their subgraphs. It is due to Kelly[1] and Ulam.[2]

Formal statements

Given a graph , a vertex-deleted subgraph of is a subgraph formed by deleting exactly one vertex from . Clearly, it is an induced subgraph of .

For a graph , the deck of G, denoted , is the multiset of all vertex-deleted subgraphs of . Each graph in is called a card. Two graphs that have the same deck are said to be hypomorphic.

With these definitions, the conjecture can be stated as:

  • Reconstruction Conjecture: Any two hypomorphic graphs on at least three vertices are isomorphic.

(The requirement that the graphs have at least three vertices is necessary because both graphs on two vertices have the same decks.)

Harary[3] suggested a stronger version of the conjecture:

  • Set Reconstruction Conjecture: Any two graphs on at least four vertices with the same sets of vertex-deleted subgraphs are isomorphic.

Given a graph , an edge-deleted subgraph of is a subgraph formed by deleting exactly one edge from .

For a graph , the edge-deck of G, denoted , is the multiset of all edge-deleted subgraphs of . Each graph in is called an edge-card.

  • Edge Reconstruction Conjecture: (Harary, 1964)[3] Any two graphs with at least four edges and having the same edge-decks are isomorphic.

Verification

Both the reconstruction and set reconstruction conjectures have been verified for all graphs with at most 11 vertices (McKay[4]).

In a probabilistic sense, it has been shown (Bollobás[5]) that almost all graphs are reconstructible. This means that the probability that a randomly chosen graph on vertices is not reconstructible goes to 0 as goes to infinity. In fact, it was shown that not only are almost all graphs reconstructible, but in fact that the entire deck is not necessary to reconstruct them — almost all graphs have the property that there exist three cards in their deck that uniquely determine the graph.

Reconstructible graph families

The conjecture has been verified for a number of infinite classes of graphs.

Recognizable properties

Template:Unreferenced-section

In context of the reconstruction conjecture, a graph property is called recognizable if one can determine the property from the deck of a graph. The following properties of graphs are recognizable:

Reduction

The reconstruction conjecture is true if all 2-connected graphs are reconstructible [8]

Other structures

It has been shown that the following are not in general reconstructible:

  • Digraphs: Infinite families of non-reconstructible digraphs are known, including tournaments (Stockmeyer[9]) and non-tournaments (Stockmeyer[10]). A tournament is reconstructible if it is not strongly connected.[11] A weaker version of the reconstruction conjecture has been conjectured for digraphs, see New digraph reconstruction conjecture.
  • Hypergraphs (Kocay[12]).
  • Infinite graphs. Let T be a tree on an infinite number of vertices such that every vertex has infinite degree. The counterexample is T and 2T. The question of reconstructibility for locally finite infinite graphs is still open.

See also

Further reading

For further information on this topic, see the survey by Nash-Williams.[13]

References

  1. Kelly, P. J., A congruence theorem for trees, Pacific J. Math. 7 (1957), 961–968.
  2. Ulam, S. M., A collection of mathematical problems, Wiley, New York, 1960.
  3. 3.0 3.1 Harary, F., On the reconstruction of a graph from a collection of subgraphs. In Theory of Graphs and its Applications (Proc. Sympos. Smolenice, 1963). Publ. House Czechoslovak Acad. Sci., Prague, 1964, pp. 47–52.
  4. McKay, B. D., Small graphs are reconstructible, Australas. J. Combin. 15 (1997), 123–126.
  5. Bollobás, B., Almost every graph has reconstruction number three, J. Graph Theory 14 (1990), 1–4.
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  7. 7.0 7.1 von Rimscha, M.: Reconstructibility and perfect graphs. Discrete Mathematics 47, 283–291 (1983)
  8. Yang Yongzhi:The reconstruction conjecture is true if all 2-connected graphs are reconstructible. Journal of graph theory 12, 237–243 (1988)
  9. Stockmeyer, P. K., The falsity of the reconstruction conjecture for tournaments, J. Graph Theory 1 (1977), 19–25.
  10. Stockmeyer, P. K., A census of non-reconstructable digraphs, I: six related families, J. Combin. Theory Ser. B 31 (1981), 232–239.
  11. Harary, F. and Palmer, E., On the problem of reconstructing a tournament from sub-tournaments, Monatsh. Math. 71 (1967), 14–23.
  12. Kocay, W. L., A family of nonreconstructible hypergraphs, J. Combin. Theory Ser. B 42 (1987), 46–63.
  13. Nash-Williams, C. St. J. A., The Reconstruction Problem, in Selected topics in graph theory, 205–236 (1978).