%% 
%%  Example 01-06-3 
%%  Copyright (C) 2017 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{158.31393pt}

%StartShownPreambleCommands
\usepackage{mathtools}% for \boxed 
%StopShownPreambleCommands
\begin{document}
\begin{equation}
\fboxsep=0pt\fboxrule=0.1pt
\boxed{\sum_{%
  {{1\le j\le p\atop {%
    {1\le j\le q\atop 1\le k\le r}}}%
  }}%
}\boxed{a_{ij}b_{jk}c_{ki}}
\end{equation}
\end{document}
