title: mathematica returned imaginary number for x^(2/3)
body: <pre><code>N[(-27)^(2/3)]//returns -4.5+7.79423i 
  </code></pre>
  <p>what would be the reason Mathematica returned an imaginary number for x^(2/3) when x= -27?</p>
  <p>it should be 9</p>
  <p>I think this is not a bug, but I do not understand the reason</p>