%%
%% The LaTeX Companion, 3ed
%%
%% Example A-1-5 on page II-626 in "Defining simple commands".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{123.0pt}
%StartShownPreambleCommands
\newcommand\xvec{\ensuremath{x_1,\ldots,x_n}}
%StopShownPreambleCommands

\begin{document}
The series of \xvec\ or $\xvec+G_{\xvec}$ \par
\renewcommand\xvec[2]{\ensuremath{#1_1,\ldots,#1_{#2}}}
The series of \xvec{x}{n} or $\xvec{y}{k}+G_{\xvec{z}{4}}$
\end{document}
