https://s0.wp.com/latex.php?latex=%5Cdisplaystyle++r_k%28m%29+%3D+%5Cfrac%7B1%7D%7B2%7D+%5Cint_%7B%5Cgamma_%7B%5Ctau_0+%5Crightarrow+%5Ctau_0%2B2%7D%7D+%5Ctheta%28%5Ctau%29%5Ek+e%5E%7B-%5Cpi+i+m+%5Ctau%7D%5C+d%5Ctau&bg=ffffff&fg=000000&s=0&c=20201002