# Topological combinatorics

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

In 1978 the situation was reversed when methods from algebraic topology were used to solve a problem in combinatorics when László Lovász proved the Kneser conjecture, thus beginning the new study 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 Frank: Given a k-connected graph G, k points v1,...,vkV(G), and k positive integers n1,n2,...,nk that sum up to |V(G)|, there exists a partition {V1,...,Vk} of V(G) such that viVi, |Vi|=ni and Vi spans a connected subgraph.

The most notable application of topological combinatorics has been to graph coloring problems. Also in 1987 the necklace problem was solved by Noga Alon. It has also been used to study complexity problems in linear decision tree algorithms and the evasiveness conjecture. Other areas include topology of partially ordered sets and bruhat orders.

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