$$U_{881}(x, y) = \frac{\left(2 - 2 \cos{\left(\frac{2 \arcsin{\left(\frac{3 \sqrt{3} \sqrt{x \left(y + 1\right)}}{2} \right)}}{3} \right)}\right) \left(y + 1\right)^{2}}{3 x}$$

$$T_{881}(n, m, k) = \frac{k {\binom{3 k + n}{m}} {\binom{2 k + 3 n - 1}{n}}}{k + n}$$