$$U_{876}(x, y) = \frac{\sqrt{1 - 4 y} \left(1 - \sqrt{- \frac{4 x}{\sqrt{1 - 4 y}} + 1}\right)}{2 x}$$

$$TA_{984}(n, k) = \begin{cases}{4}^{n} \binom {n+j-1} {n} &\text{if k even, where j =} \frac {k} {2},\\\frac {\binom {n+j} {n} \binom {2n+2j} {n+j}} {\binom {2j} {j}}&\text{if k odd, where j =} \frac {k-1} {2},\end{cases} $$$$T_{876}(n, m, k) = \frac{k \operatorname{TA_{984}}{\left(m,n \right)} {\binom{k + 2 n - 1}{n}}}{k + n}$$