You can write the symbol of a number set (with double lines) using the `mathbb`

font which is supported by `amsfonts`

or `amssymb`

package.

In order to write the notation of a set of real numbers, you can thus use

\mathbb{R}

in math mode if you have included the correct package, i.e. if you either have

\usepackage{amsfonts}

or

\usepackage{amssymb}

in the preamble of your document, so you can write stuff like the following:

$\mathbb{N}$ is the set of natural numbers, $\mathbb{Z}$ is the set of integers, $\mathbb{Q}$ is the set of rational numbers and $\mathbb{R}$ is the set of real numbers.

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