$$U_{698}(x, y) = \frac{y \left(1 - \sqrt{- \frac{2 x \left(1 - \sqrt{1 - 4 y}\right)}{y} + 1}\right)}{x \left(1 - \sqrt{1 - 4 y}\right)}$$

$$T_{698}(n, m, k) = \begin{cases}1&\text{if n+m=0} ,\ \\\frac{k n {\binom{k + 2 n - 1}{n}} {\binom{2 m + n - 1}{m}}}{\left(k + n\right) \left(m + n\right)} \end{cases} $$