# Schur's theorem

In discrete mathematics, Schur's theorem is either of two different theorems of the mathematician Issai Schur. In differential geometry, Schur's theorem is a theorem of Axel Schur. In functional analysis, Schur's theorem is often called Schur's property, also due to Issai Schur.

## Ramsey theory

In Ramsey theory, Schur's theorem states that for any partition of the positive integers into a finite number of parts, one of the parts contains three integers x, y, z with

$x + y = z.\$

Moreover, for every positive integer c, there exists a number S(c), called Schur's number, such that for every partition of the integers

$\{1, ..., S(c)\}\$

into c parts, one of the parts contains integers x, y, and z with

$x + y = z.\$

Folkman's theorem generalizes Schur's theorem by stating that there exist arbitrarily large sets of integers all of whose nonempty sums belong to the same part.

## Combinatorics

In combinatorics, Schur's theorem tells the number of ways for expressing a given number as a (non-negative, integer) linear combination of a fixed set of relatively prime numbers. In particular, if $\{a_1,\ldots,a_n\}$ is a set of integers such that $gcd(a_1,\ldots,a_n)=1$, the number of different tuples of non-negative integer numbers $(c_1,\ldots,c_n)$ such that $x=c_1a_1 + \cdots + c_na_n$ when $x$ goes to infinity is:

$\frac{x^{n-1}}{(n-1)!a_1\ldots a_n}(1+o(1)).$

As a result, for every set of relatively prime numbers $\{a_1,\ldots,a_n\}$ there exists a value of $x$ such that every larger number is representable as a linear combination of $\{a_1,\ldots,a_n\}$ in at least one way. This consequence of the theorem can be recast in a familiar context considering the problem of changing an amount using a set of coins. If the denominations of the coins are relatively prime numbers (such as 2 and 5) then any sufficiently large amount can be changed using only these coins. (See Coin problem.)

## Differential geometry

In differential geometry, Schur's theorem compares the distance between the endpoints of a space curve $C^*$ to the distance between the endpoints of a corresponding plane curve $C$ of less curvature.

Suppose $C(s)$ is a plane curve with curvature $\kappa(s)$ which makes a convex curve when closed by the chord connecting its endpoints, and $C^*(s)$ is a curve of the same length with curvature $\kappa^*(s)$. Let $d$ denote the distance between the endpoints of $C$ and $d^*$ denote the distance between the endpoints of $C^*$. If $\kappa^*(s) \leq \kappa(s)$ then $d^* \geq d$.

Schur's theorem is usually stated for $C^2$ curves, but John M. Sullivan has observed that Schur's theorem applies to curves of finite total curvature (the statement is slightly different).

## Linear Algebra

Main article: Schur decomposition

In linear algebra Schur’s theorem is referred to as either the triangularization of a square matrix with complex entries, or of a square matrix with real entries and real eigenvalues.

## Functional analysis

In functional analysis and the study of Banach spaces, Schur's theorem, due to J. Schur, often refers to Schur's property, that for certain spaces, weak convergence implies convergence in the norm.