Yesterday, the best answer is: -4e^x/x^2.  (Contributed by turtle. Good Job!)
turtle solved the puzzle at 20:30:35.758; length: 15
turtle solved the puzzle at 20:31:37.129; length: 13
fire solved the puzzle at 22:05:32.946; length: 15
fire solved the puzzle at 22:06:15.396; length: 13