$$U_{855}(x, y) = \frac{\sqrt{3} \left(2 y + 2\right) \sin{\left(\frac{\operatorname{asin}{\left(\frac{3 \sqrt{3} \sqrt{x \left(y + 1\right)}}{2} \right)}}{3} \right)}}{3 \sqrt{x \left(y + 1\right)}}$$

$$T_{855}(n, m, k) = \frac{k {\binom{k + n}{m}} {\binom{k + 3 n - 1}{n}}}{k + 2 n}$$