$$U_{990}(x, y) = \frac{1 - \sqrt{- 4 x \left(y + 1\right) + 1}}{2 x \left(y + 1\right)^{3}}$$

$$T_{990}(n, m, k) = \frac{k {\binom{- 2 k + n}{m}} {\binom{k + 2 n - 1}{n}}}{k + n}$$