# Erdős–Fuchs theorem

In mathematics, in the area of combinatorial number theory, the Erdős–Fuchs theorem is a statement about the number of ways that numbers can be represented as a sum of two elements of a given set, stating that the average order of this number cannot be close to being a linear function.

The theorem is named after Paul Erdős and Wolfgang Heinrich Johannes Fuchs, who published it in 1956.

## Statement

Let A be a subset of the natural numbers and r(n) denote the number of ways that a natural number n can be expressed as the sum of two elements of A (taking order into account). We consider the average

${\displaystyle R(n)={\frac {r(1)+r(2)+\cdots +r(n)}{n}}.}$

The theorem states that

${\displaystyle R(n)=C+O\left(n^{-3/4-\varepsilon }\right)}$

cannot hold unless C = 0.