# Combinatorics

- kind: region

Counting, coloring and arrangement. The Erdős problems live mostly here, and so do the results most often settled by computer search and SAT solvers.

Mathematics Subject Classification: 05, 06.

## Problems

- [Four color theorem](/wiki/Problem:four-color-theorem.md) (solved)
- [Ramsey number R(5,5)](/wiki/Problem:ramsey-r55.md) (open)
- [Hadwiger–Nelson problem](/wiki/Problem:hadwiger-nelson.md) (partial)
- [Union-closed sets conjecture](/wiki/Problem:union-closed-sets.md) (partial)
- [Boolean Pythagorean triples problem](/wiki/Problem:boolean-pythagorean-triples.md) (solved)
- [Kahn–Kalai conjecture](/wiki/Problem:kahn-kalai.md) (solved)

Part of [Mathematics](/wiki/Continent:mathematics.md).

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