https://s0.wp.com/latex.php?latex=%5Crho%28%5Ctheta%29+%3D+%5Cfrac%7B1%7D%7B2%7D+%28+%5Cvert+%5Ctheta+%5Crangle+%5Clangle+%5Ctheta+%5Cvert+%2B+%5Cvert+-%5Ctheta+%5Crangle+%5Clangle+-%5Ctheta+%5Cvert++%29+&bg=ffffff&fg=404040&s=1