Topological combinatorics

Mathematical subject

The mathematical discipline of topological combinatorics is the application of topological and algebro-topological methods to solving problems in combinatorics.

## History

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The discipline of combinatorial topology used combinatorial concepts in topology and in the early 20th century this turned into the field of algebraic topology.

In 1978 the situation was reversed—methods from algebraic topology were used to solve a problem in combinatorics—when László Lovász proved the Kneser conjecture, thus beginning the new field of topological combinatorics. Lovász's proof used the Borsuk–Ulam theorem and this theorem retains a prominent role in this new field. This theorem has many equivalent versions and analogs and has been used in the study of fair division problems.

In another application of homological methods to graph theory, Lovász proved both the undirected and directed versions of a conjecture of András Frank: Given a k-connected graph G, k points  v 1 , … , v k ∈ V ( G ) {\displaystyle v_{1},\ldots ,v_{k}\in V(G)} , and k positive integers n 1 , n 2 , … , n k {\displaystyle n_{1},n_{2},\ldots ,n_{k}} that sum up to  | V ( G ) | {\displaystyle |V(G)|} , there exists a partition  { V 1 , … , V k } {\displaystyle \\{V_{1},\ldots ,V_{k}\\}} of  V ( G ) {\displaystyle V(G)} such that  v i ∈ V i {\displaystyle v_{i}\in V_{i}} ,  | V i | = n i {\displaystyle |V_{i}|=n_{i}} , and  V i {\displaystyle V_{i}} spans a connected subgraph.

In 1987 the necklace splitting problem was solved by Noga Alon using the Borsuk–Ulam theorem. It has also been used to study complexity problems in linear decision tree algorithms and the Aanderaa–Karp–Rosenberg conjecture. Other areas include topology of partially ordered sets and Bruhat orders.

Additionally, methods from differential topology now have a combinatorial analog in discrete Morse theory.

## See also

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* Sperner's lemma
* Discrete exterior calculus
* Topological graph theory
* Combinatorial topology
* Finite topological space

## References

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* de Longueville, Mark (2004), "25 years proof of the Kneser conjecture - The advent of topological combinatorics" (PDF), EMS Newsletter, Southampton, Hampshire: European Mathematical Society, pp. 16–19, retrieved 2008-07-29.

## Further reading

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* Björner, Anders (1995), "Topological Methods", in Graham, Ronald L.; Grötschel, Martin; Lovász, László (eds.), Handbook of Combinatorics (PDF), vol. 2, The MIT press, ISBN 978-0-262-07171-0.
* Kozlov, Dmitry (2005), Trends in topological combinatorics, arXiv:math.AT/0507390, Bibcode:2005math......7390K.
* Kozlov, Dmitry (2007), Combinatorial Algebraic Topology, Springer, ISBN 978-3-540-71961-8.
* Lange, Carsten (2005), Combinatorial Curvatures, Group Actions, and Colourings: Aspects of Topological Combinatorics (PDF), Ph.D. thesis, Technische Universität Berlin.
* Matoušek, Jiří (2003), Using the Borsuk-Ulam Theorem: Lectures on Topological Methods in Combinatorics and Geometry, Springer, ISBN 978-3-540-00362-5.
* Barmak, Jonathan (2011), Algebraic Topology of Finite Topological Spaces and Applications, Springer, ISBN 978-3-642-22002-9.
* de Longueville, Mark (2011), A Course in Topological Combinatorics, Springer, ISBN 978-1-4419-7909-4.