Try using substack:

\usepackage{amsmath}

...

\begin{equation}
  \prod_{\substack{
    1 \le i \le n \\
    1 \le j \le m} }
  M_{i,j}
\end{equation}


This example is taken from the Latex Wikibook.

LaTeX command for double lower index summation
Everything in Java seems to follow capitalization rules except for Hashtable. 

Hashtable&lt;String, String&gt; ht = new Hashtable&lt;String, String&gt;();


as opposed to 

ArrayList&lt;String&gt; a = new ArrayList&lt;String&gt;();


or 

HashMap&lt;String,Integer&gt; a = new HashMap&lt;String,Integer&gt;(); 


Why is this ? Is Hash Table read as just one word (Hashtable) ?
