## Theorem

The set of real numbers $\R$ is closed under addition:

$\forall x, y \in \R: x + y \in \R$

## Proof

From the definition, the real numbers are the set of all equivalence classes $\eqclass {\sequence {x_n} } {}$ of Cauchy sequences of rational numbers.

Let $x = \eqclass {\sequence {x_n} } {}, y = \eqclass {\sequence {y_n} } {}$, where $\eqclass {\sequence {x_n} } {}$ and $\eqclass {\sequence {y_n} } {}$ are such equivalence classes.

From the definition of real addition, $x + y$ is defined as:

$\eqclass {\sequence {x_n} } {} + \eqclass {\sequence {y_n} } {} = \eqclass {\sequence {x_n + y_n} } {}$

We have that $\forall i \in \N: x_i \in \Q, y_i \in \Q$, therefore $x_i + y_i \in \Q$.

So it follows that $\eqclass {\sequence {x_n + y_n} } {} \in \R$.

$\blacksquare$