# Undefined Control Sequence when using $\R$

Using the following packages;

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage{fancyhdr}
\usepackage{amsthm}
\usepackage{float}
\usepackage[round]{natbib}
\usepackage{lastpage}
\usepackage{indentfirst}
\usepackage{graphicx}
\usepackage{amsmath}


It says undefined control sequence when writing

{It is called a simple arc of a curve, the set $C$ of points $M(x,y) \in \text{$\R$} ^2$
(two-dimensional Euclidean space) satisfying the equation:}


also changing this to the following doesn't work

{It is called a simple arc of a curve, the set $C$ of points $M(x,y) \in \R ^2$


I have included

\usepackage{amsmath}


like it says to do to fix the issue on most forums

• And where is \R defined? Please, make complete small document that we can tested it. Welcome to the site! – Zarko Nov 19 '16 at 23:01

\end{document}


In other news, I'm not quite sure why you seem to have braces around your sentence, in general they would not be needed so I've removed them here.

• Many people use double letters for this reason, like \RR, \CC, &c. And some purists prefer to define them as \mathbf{R}, \mathbf{C},… – Bernard Nov 19 '16 at 23:21