Yesterday, the best answer is: 2^((x^2(x+1))^-0.5). (Contributed by Gaaae. Good Job!)

unnamed solved the puzzle at 21:57:30.735; length: 18

TurtleLink solved the puzzle at 02:01:59.205; length: 16

tangentz solved the puzzle at 07:16:31.307; length: 18