https://s0.wp.com/latex.php?latex=%5Cdisplaystyle+%5Csum_%7Bn%3D0%7D%5E%7B%5Cinfty%7DU_n%28x%29+t%5En+%3D+%5Cfrac%7B1%7D%7B1-2+t+x%2Bt%5E2%7D.&bg=ffffff&fg=000000&s=0