%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-4-10 on page II-168 in "Stacking in subscripts and superscripts".
%%
%% 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{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{135.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.2\abovedisplayskip}%
    \setlength\belowdisplayskip{.2\belowdisplayskip}%
  }

%StartShownPreambleCommands
\usepackage{amsmath}
%StopShownPreambleCommands

\begin{document}
\begin{gather}
  \sum_{\substack{0 \le i \le m \\ 0 < j < n}} P(i, j) \\
  \sum_{\begin{subarray}{l}  i \in \Lambda   \\
                             0 \le i \le m   \\
                             0 < j < n
        \end{subarray}}  P(i, j)
\end{gather}
\end{document}