$$U_{786}(x, y) = \left(y + 1\right) \left(\frac{x}{y + 1} + 1\right)^{3}$$

$$T_{786}(n, m, k) = \begin{cases}{\binom{k - n}{m}}&\text{if n=0} ,\ \\\frac{3 \left(-1\right)^{n - 1} k {\binom{k - n}{m}} {\binom{- 3 k + n - 1}{n - 1}}}{n} \end{cases} $$