# Schur's Theorem (Ramsey Theory)

## Contents

## Theorem

Let $r$ be a positive integer.

Then there exists a positive integer $S$ such that:

- for every partition of the integers $\left\{{1, \ldots, S}\right\}$ into $r$ parts, one of the parts contains integers $x$, $y$ and $z$ such that:
- $x + y = z$

## Proof

Let:

- $n = R \left({3, \ldots, 3}\right)$

where $R \left({3, \ldots, 3}\right)$ denotes the Ramsey number on $r$ colors.

Take $S$ to be $n$.

partition the integers $\left\{{1, \ldots, n}\right\}$ into $r$ parts, which we denote by **colors**.

That is:

- The integers in the first part are said to be
**colored**$c_1$ - The integers in the second part are said to be colored $c_2$

... and so on till color $c_r$.

Thus $\left\{{1, \ldots, S}\right\}$ has been **$r$-colored**.

(This terminology is common in Ramsey theory.)

Now consider the complete graph $K_n$.

Now color the edges of $K_n$ as follows:

}

From the definition of $R \left({3, \ldots, 3}\right)$ and Ramsey's Theorem, $K_n$ will definitely contain a monochromatic triangle, say built out of the vertices $i > j > k$.

Let the triangle be colored $c_m$.

Now $i - j$, $i - k$ and $j - k$ will also be colored $c_m$.

That is, $i - j$, $i - k$ and $j - k$ will belong to the same part in the partition.

It only remains to take $x = i - j$, $y = j - k$ and $z = i - k$ to complete the proof.

$\blacksquare$

## An extension

The above proving technique allows to obtain a variety of similar and further going results. Here is just a sample:

**THEOREM 1**

Let $r$ be a positive integer.

Then there exists a positive integer $S$ such that:

- for every partition of the integers $\left\{{1, \ldots, S}\right\}$ into $r$ parts, one of the parts contains integers $x$, $y$ and $z$ such that:
- $x + y = z$

- and:
- $x \ne y$

This theorem follows from the following one:

**THEOREM 2**

Let $r$ be a positive integer.

Then there exists a positive integer $S$ such that: for every partition of the integers $\left\{{1, \ldots, S}\right\}$ into $r$ parts, one of the parts contains integers $a$, $b$, $a + b$, $c$, $b + c$, and $d$ such that:

- $a + b + c = d$

**PROOF**

The proof is nearly the same as of the original Schur's theorem above, except that one uses $R \left({4, \ldots, 4}\right)$.

$\blacksquare$

## Source of Name

This entry was named for Issai Schur.