Yesterday, the best answer is: e^(2x).  (Contributed by dnialh. Good Job!)
fire solved the puzzle at 20:36:15.993; length: 12
dmialh solved the puzzle at 23:52:24.906; length: 12
dmialh solved the puzzle at 23:52:48.842; length: 12
dmialh solved the puzzle at 23:53:08.032; length: 12
anid solved the puzzle at 18:01:09.325; length: 10
dnialh solved the puzzle at 19:44:40.799; length: 12
dnialh solved the puzzle at 19:53:28.790; length: 10