https://s0.wp.com/latex.php?latex=%5Cdelta+S%3DT%5Cint_%7B%5Ctau_i%7D%5E%7B%5Ctau_f%7Dd%5Ctau%5Cint_%7B0%7D%5E%7Bl%7Dd%5Csigma+%5Cdelta+X%5E%5Cmu%5Cleft%28%5Cdfrac%7B%5Cpartial+P%5E%5Ctau_%5Cmu%7D%7B%5Cpartial+%5Ctau%7D%2B%5Cdfrac%7B%5Cpartial+P%5E%5Csigma_%5Cmu%7D%7B%5Cpartial%5Csigma%7D%5Cright%29%3D0&bg=ffffff&fg=333333&s=0