Yesterday, the best answer is: 2e^(2x)x.  (Contributed by fire. Good Job!)
fire solved the puzzle at 20:06:38.246; length: 19
dnialh solved the puzzle at 20:11:15.791; length: 18
dnialh solved the puzzle at 20:11:46.975; length: 17
dnialh solved the puzzle at 14:32:36.246; length: 17