$$U_{864}(x, y) = \frac{2 \sqrt{3} \sin{\left(\frac{\arcsin{\left(\frac{3 \sqrt{3} \sqrt{\frac{x}{1 - y}}}{2} \right)}}{3} \right)}}{3 \sqrt{\frac{x}{1 - y}} \left(1 - y\right)}$$

$$T_{864}(n, m, k) = \frac{k {\binom{k + 3 n - 1}{n}} {\binom{k + m + n - 1}{m}}}{k + 2 n}$$