Real number(ℝ) symbol in LaTeX

The real number symbol is represented by R’s bold font-weight or typestyle blackboard bold. However, in most cases the type-style of capital letter R is blackboard-bold.

To do this, you need to have \mathbb{R} command that is present in multiple packages. For example amsmath, amssymb, texfonts, and pxfonts.

\documentclass{article}
\usepackage{amssymb}
\begin{document}
  $$ \mathbb{R}\in \left[a_1,a_n\right] $$
  $$ \mathbb{R}\in \left[S_1,S_n\right] $$
  $$ \mathbb{R}_1, \mathbb{R}^n,\mathbb{R}_{-*},\mathbb{R}_{+*} $$
\end{document}

Output :

Positive real number and Negative real number symbols are denoted by ℝ⁺ and ℝ^–. Which, you can easily represent using the superscript with the \mathbb command.

\documentclass{article}
\usepackage{amssymb}
\begin{document}
   $$ \mathbb{R}_{\geq 0} \in \mathbb{R}^+ \cup \{0\} $$
   $$ 0 < \mathbb{R}^+ < \infty $$
   $$ -\infty < \mathbb{R}^- <0 $$
   $$ \mathbb{R}_0 = \{0\} $$ $$ 
\end{document}

Output :