title: What is following time complexity?
body: <p>If a for loop is defined as
  <code>for (int i = 2; i &lt; n; i = i*i + i)
  </code> What the
  "i*2+i" represents for time complexity.
  What is time complexity in big-O notation? 
  How could I solve big-O notation for this increasing index?
  Ex: i = 2 , 6 , 42 , 1086 , .... (general formula "i*2+i") </p>