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
Best Answer
To expand on the excellent accepted answer: There are many alternatives to
amsfonts
for this symbol.In the modern toolchain, there is no need to load any additional packages beyond
unicode-math
to get a ℝ that looks like the one from the early answer. It is already set up by default.However, you can load the double-struck alphabet from any math font, or any display font that you’d rather use, with the
range=
option of\setmathfont
. For example, to use Typoliner:If you need compatibility with PDFLaTeX, or you would rather use a legacy NFSS font, you might want to use the
mathalfa
package to select your supplemental math alphabets.