https://s0.wp.com/latex.php?latex=S%28f%29+%3D+%5Csqrt%7B2%5Cpi+l%5E2%7D%5Cexp%28-2%5Cpi%5E2+l%5E2+f%5E2%29&bg=ffffff&fg=7f8d8c&s=0&c=20201002