$$U_{784}(x, y) = \frac{\left(- 2 y - \sqrt{1 - 4 y} + 1\right)^{3}}{8 y^{6} \left(- x + \frac{- 2 y - \sqrt{1 - 4 y} + 1}{2 y^{2}}\right)^{2}}$$

$$T_{784}(n, m, k) = \begin{cases}{\binom{2 k + n - 1}{n}}&\text{if m=0} ,\ \\\frac{2 \left(k - n\right) {\binom{2 k + n - 1}{n}} {\binom{2 k + 2 m - 2 n - 1}{m - 1}}}{m} \end{cases} $$