%%
%% The LaTeX Companion, 2ed (second printing August 2004)
%%
%% Example A-1-5 on page 845.
%%
%% 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\lvec[2][n]
 {\ensuremath{#2_1+\cdots + #2_{#1}}}
\StopShownPreambleCommands
\begin{document}
For the series \lvec{x}  we have
\[ \lvec{x} = \sum_{k=1}^{n} G_{\lvec[k]{y}} \]
\end{document}