$$U_{991}(x, y) = \frac{- 2 x \left(y + 1\right) - \sqrt{- 4 x \left(y + 1\right) + 1} + 1}{2 x^{2} \left(y + 1\right)^{2}}$$

$$T_{991}(n, m, k) = \frac{k {\binom{n}{m}} {\binom{2 k + 2 n}{n}}}{k + n}$$