Using unicode ℝ as commands \mathbb{R}
\documentclass{article}
\usepackage{amssymb}
\usepackage[utf8]{inputenc}
\usepackage{newunicodechar}
\newunicodechar{ℝ}{\mathbb{R}}
\begin{document}
$ \mathbb{R} =1$
$ ℝ = 1 $
\end{document}
For XeLaTeX you can just remove the inputenc
package, but in this case is simpler the David's suggestion of use unicode-math
.
If you are using
\usepackage[utf8]{inputenc}
then just add
\DeclareUnicodeCharacter{211D}{\mathbb{R}}
With unicode TeX engines it is probably advisable to use unicode-math
which would allow ℝ to be typeset directly, but if you are using 8bit math fonts still then you can use
\catcode`ℝ=13
\newcommandℝ{\mathbb{R}}