$$U_{216}(x, y) = \frac{\sqrt{1 - 4 y} - \sqrt{- 4 x \sqrt{1 - 4 y} - 4 y + 1}}{2 x \sqrt{1 - 4 y}}$$

$$T_{216}(n, m, k) = \begin{cases}\frac{k {\binom{\frac{k}{2} + m + \frac{n}{2}}{m}} {\binom{k + 2 m + n}{\frac{k}{2} + m + \frac{n}{2}}} {\binom{k + 2 n - 1}{n}}}{\left(k + n\right) {\binom{k + n}{\frac{k}{2} + \frac{n}{2}}}}&\text{if k even} ,\ \\\frac{k {\binom{k + 2 n - 1}{n}} {\binom{\frac{k}{2} + m + \frac{n}{2} - \frac{1}{2}}{m}} {\binom{k + 2 m + n - 1}{\frac{k}{2} + m + \frac{n}{2} - \frac{1}{2}}}}{\left(k + n\right) {\binom{k + n - 1}{\frac{k}{2} + \frac{n}{2} - \frac{1}{2}}}}&\text{if k odd} \end{cases} $$