title: Values for 'fun= ' in *apply functions
body: <p>The *apply (apply, tapply, lapply etc.) are a very useful and straightforward method of applying a function across an array of data, with the generic structure:</p>
  
  <pre><code>*apply(data, function ...)
  </code></pre>
  
  <p>However, I cannot find a list of valid functions. </p>
  
  <p>Can anyone provide a link to such a list?</p>