title: Have I encountered a limit to mpmath's floating-point precision?
body: <p><a href="http://mpmath.org/" rel="nofollow"><code>mpmath</code></a> purports to support "arbitrary-precision floating-point arithmetic."</p>
  
  <p>And yet . . .</p>
  
  <pre><code>&gt;&gt;&gt; import mpmath
  &gt;&gt;&gt; 1 + mpmath.erf(-5.921)
  mpf('1.1102230246251565e-16')
  &gt;&gt;&gt; 1 + mpmath.erf(-5.922)  # I expect a smaller positive number here.
  mpf('0.0')
  </code></pre>
  
  <p>Am I missing something?  Or is this a fundamental limitation of <code>mpmath</code>?</p>
  
  <p>@jonrsharpe has suggested that the problem is that I've submitted a <code>float</code> to <code>erf</code>.  However, the code below shows that this is not the problem:</p>
  
  <pre><code>&gt;&gt;&gt; 1 + mpmath.erf(mpmath.mpf('-5.922'))
  mpf('0.0')
  </code></pre>