$$U_{88}(x, y) = \frac{\sqrt{1 - 4 x} - \sqrt{- 4 x - 4 y + 1}}{2 y}$$

$$T_{88}(n, m, k) = \begin{cases}1&\text{if n=0,m=0} ,\ \\\frac{4^{n} k {\binom{k + 2 m - 1}{m}} {\binom{\frac{k}{2} + m + n - 1}{n}}}{k + m} \end{cases} $$