title: How to put a symbol above another in LaTeX?
body: <p>How to put a symbol above (on the head of) another? For example, I want to produce something like this in one line. </p>
  
  <pre><code>a
  #
  </code></pre>
  
  <p>i.e., <code>a</code> above <code>#</code>. </p>
  
  <p>The effect should be almost the same with <code>$#^a$</code> except that <code>a</code> is on the top instead of top right of <code>#</code>.</p>