Double subscript error using tensor package

Question

I have been attempting to typeset tensors using the tensor package and have come across an error when attempting to create subscripts on the subscripts of my tensors.

An example of this would be the following:

\documentclass{report}
\usepackage{mathtools}
\usepackage{tensor}

\begin{document}

\begin{equation} \label{tensor-symmetrize}
T\indices{_{(\mu_1 \mu_2 \cdots \mu_n)}_\rho^\sigma} = \frac{1}{n!} (T \indices{_{\mu_1}_{\mu_2}}_{\cdots}_{\mu_n}_\rho^\sigma 
+ \text{sum over permutations of indices}) .
\end{equation}

\end{document}

On compiling this document I get an error saying

! Double subscript.
l.8 ...frac{1}{n!} (T \indices{_{\mu_1}_{\mu_2}}_
                                              {\cdots}_{\mu_n}_\rho^\sig...

Answer 1

You have a misbalance in braces:

\begin{equation} \label{tensor-symmetrize}
T\indices{_{(\mu_1 \mu_2 \cdots \mu_n)}_\rho^\sigma} = \frac{1}{n!} (T \indices{_{\mu_1}_{\mu_2}_{\cdots}_{\mu_n}_\rho^\sigma }
+ \text{sum over permutations of indices}) .
\end{equation}

You could take out the extra braces and subscripts on the right-hand side like this:

\begin{equation} 
T\indices{_{(\mu_1 \mu_2 \cdots \mu_n)}_\rho^\sigma} = \frac{1}{n!} (T \indices{_{\mu_1\mu_2\cdots\mu_n}_\rho^\sigma }
+ \text{sum over permutations of indices})
\end{equation}

Answer 2

Another way is hiding the "inner indices".

\documentclass[a4paper,11pt]{report}

\usepackage{amsthm}
\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{tensor}

\newcommand{\cind}[2]{#1_{#2}}

\begin{document}

\begin{equation} \label{tensor-symmetrize}
T\indices{_{(\cind{\mu}{1} \cind{\mu}{2} \cdots \cind{\mu}{n})}_\rho^\sigma} =
\frac{1}{n!} (T \indices{_{\cind{\mu}{1}}_{\cind{\mu}{2}}_{\cdots}_{\cind{\mu}{n}}_\rho^\sigma}
+ \text{sum over permutations of indices}) .
\end{equation}

\end{document}