%%
%% The LaTeX Companion, 2ed (second printing August 2004)
%%
%% Example A-1-4 on page 844.
%%
%% Copyright (C) 2004 Frank Mittelbach, Michel Goossens,
%%    Johannes Braams, David Carlisle, and Chris Rowley
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3
%% of this license or (at your option) any later version.
%%
%% See http://www.latex-project.org/lppl.txt for details.
%%

\documentclass{ttctexa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{183.0pt}
\StartShownPreambleCommands
\newcommand\xvec{\ensuremath{x_1,\ldots,x_n}}
\StopShownPreambleCommands
\begin{document}
The series of \xvec\ or $\xvec+G_{\xvec}$ \par
\renewcommand\xvec[1]{\ensuremath{x_1,\ldots,x_{#1}}}
The series of \xvec{n} or $\xvec{k}+G_{\xvec{k}}$
\end{document}