Yesterday, the best answer is: x^2+2^sin(x).  (Contributed by dnialh. Good Job!)
fire solved the puzzle at 20:16:43.321; length: 14
dnialh solved the puzzle at 22:56:54.959; length: 12
dnialh solved the puzzle at 22:57:03.274; length: 13
dnialh solved the puzzle at 22:57:05.042; length: 13