$$U_{891}(x, y) = \frac{2 - 2 \cos{\left(\frac{2 \operatorname{asin}{\left(\frac{3 \sqrt{3} \sqrt{\frac{x}{1 - y}}}{2} \right)}}{3} \right)}}{3 x}$$

$$T_{891}(n, m, k) = \frac{k {\binom{2 k + 3 n - 1}{n}} {\binom{k + m + n - 1}{m}}}{k + n}$$