Yesterday, the best answer is: 2^(sin(x)-1).  (Contributed by turtle. Good Job!)
turtle solved the puzzle at 20:07:45.830; length: 16
turtle solved the puzzle at 20:07:47.346; length: 16
fire solved the puzzle at 20:25:44.663; length: 17
fire solved the puzzle at 20:30:19.119; length: 16
fire solved the puzzle at 20:31:51.944; length: 15