https://s0.wp.com/latex.php?latex=%5Cdisplaystyle+%5Cint_0%5E1+%5Cint_0%5E1+%7CS%28x%2Cy%29%7C%5E%7B%5Csigma%7D+dx+dy+%5Cgg+N%5E%7B%5Csigma+-+k+-+1%7D.&bg=ffffff&fg=000000&s=0&c=20201002