title: What is fixed-parameter tractability? Why is it useful?
body: <p>Some problems that are NP-hard are also <a href="http://en.wikipedia.org/wiki/Parameterized_complexity#FPT">fixed-parameter tractable</a>, or FPT. Wikipedia describes a problem as fixed-parameter tractable if there's an algorithm that solves it in time f(k) &middot; |x|<sup>O(1)</sup>.</p>
  
  <p>What does this mean? Why is this concept useful?</p>