What is causing undefined control sequence

I'm getting this error:

Undefined control sequence
1.67 ....ideology function $v_1 : A \to \R$, that represents....

which is from:

$v_i : A \to \R$

And this is my header:

\documentclass[smallextended]{svjour3}
\usepackage{latexsym}
\usepackage{graphicx} 
\usepackage{amssymb, amsmath,endnotes}
\usepackage{rotating}
\usepackage{color}
\usepackage[T1]{fontenc}
\usepackage{geometry}
\usepackage{tikz}
\usepackage{pgfplots}
\usepackage{tikz-3dplot}

Can somebody help me out with this one.