# Ramsey theory

This article provides an introduction. For a more detailed and technical article, see Ramsey's theorem.

Ramsey theory, named for Frank P. Ramsey, is a branch of mathematics that studies the conditions under which order must appear. Problems in Ramsey theory typically ask a question of the form: how many elements of some structure must there be to guarantee that a particular property will hold?

Complete disorder is impossible.

## Examples

Suppose, for example, that we know that n pigeons have been housed in m pigeonholes. How big must n be before we can be sure that at least one pigeonhole houses at least two pigeons? The answer is the pigeonhole principle: if n > m, then at least one pigeonhole will have at least two pigeons in it. Ramsey's theory generalizes this principle as explained below.

A typical result in Ramsey theory starts with some mathematical structure that is then cut into pieces. How big must the original structure be in order to ensure that at least one of the pieces has a given interesting property?

For example, consider a complete graph of order n; that is, there are n vertices and each vertex is connected to every other vertex by an edge. A complete graph of order 3 is called a triangle. Now color every edge red or blue. How large must n be in order to ensure that there is either a blue triangle or a red triangle? It turns out that the answer is 6. See the article on Ramsey's theorem for a rigorous proof.

Another way to express this result is as follows: at any party with at least six people, there are either three people who are all mutual acquaintances (each one knows the other two) or mutual strangers (each one does not know either of the other two). See theorem on friends and strangers.

This also is a special case of Ramsey's theorem, which says that for any given integer c, any given integers n1,...,nc, there is a number, R(n1,...,nc), such that if the edges of a complete graph of order R(n1,...,nc) are colored with c different colors, then for some i between 1 and c, it must contain a complete subgraph of order ni whose edges are all color i. The special case above has c = 2 and n1 = n2 = 3.

## Results

Two other key theorems of Ramsey theory are:

• Van der Waerden's theorem: For any given c and n, there is a number V, such that if V consecutive numbers are colored with c different colors, then it must contain an arithmetic progression of length n whose elements are all the same color.
• Hales-Jewett theorem: For any given n and c, there is a number H such that if the cells of a H-dimensional n×n×n×...×n cube are colored with c colors, there must be one row, column, etc. of length n all of whose cells are the same color. That is, if you play on a board with sufficiently many dimensions, then multi-player n-in-a-row tic-tac-toe cannot end in a draw, no matter how large n is, and no matter how many people are playing. Hales-Jewett theorem implies Van der Waerden's theorem.

A theorem similar to van der Waerden's theorem is Schur's theorem: for any given c there is a number N, such that if the numbers 1, 2, ..., N are colored with c different colors, then there must be a pair of integers x, y such that x, y, and x+y are all the same color. Many generalizations of this theorem exist, including Rado's Theorem, Rado-Folkman-Sanders theorem, Hindman's theorem, and the Milliken-Taylor theorem. A classic reference for these and many other results in Ramsey theory is [1].

Results in Ramsey theory typically have two notable characteristics. Firstly, they are often non-constructive; they may show that some structure exists, but they give no recipe for how to actually find this structure (other than brute force search); for instance, the pigeonhole principle is of this form. Secondly, while Ramsey theory results do say that sufficiently large objects must necessarily contain a given structure, often the proof of these results requires these objects to be enormously large - bounds that grow exponentially, or even as fast as the Ackermann function are not uncommon. In many cases these bounds are artifacts of the proof, and it is not known whether they can be substantially improved. In other cases it is known that any bound must be extraordinarily large, sometimes even larger than any primitive recursive function; see the Paris-Harrington theorem for an example.

## Ramsey-type and Turán-type results

Theorems in Ramsey theory are generally one of the two flavors. Many theorems, which are modeled after Ramsey's theorem itself, assert that in every partition of a large structured object, one of the classes necessarily contains a large structured subobject. Occasionally, the reason behind such Ramsey-type results is that the largest partition class always contains the desired substructure. A notable example is Szemerédi's theorem which is such a strengthening of van der Waerden's theorem. Another example is the density version of Hales-Jewett theorem.[2] The results of this kind are called either density results or Turán-type result, after Turán's theorem.

## Notes

1. ^ "S. A. Soman" (August 21, 2008). "Computational Methods for Large Sparse Power Systems Analysis". pp. 31.
2. ^ Hillel Furstenberg, Yitzhak Katznelson, A density version of the Hales–Jewett theorem, J. d'Analyse Math. 57 (1991), 64--119.

## References

• R. Graham, B. Rothschild, J.H. Spencer, Ramsey Theory, John Wiley and Sons, NY (1990)
• Landman and A. Robertson, Ramsey Theory on the Integers, Student Mathematical Library Vol. 24, AMS (2004)