https://s0.wp.com/latex.php?latex=%5Cdisplaystyle+%5Csum_%7Bx+%5Cleq+n+%3C+y%7D+f%28n%29+%5Cchi%28n%29+%3D+O%28+q%28%7Cf%28x%29%7C+%2B+%7Cf%28y%29%7C%29+%29.&bg=ffffff&fg=000000&s=0&c=20201002