$$U_{1281}(x, y) = \frac{\left(1 - \sqrt{- 4 x \left(y + 1\right)^{3} + 1}\right)^{3}}{8 x^{3} \left(y + 1\right)^{6}}$$

$$T_{1281}(n, m, k) = \frac{3 k {\binom{3 k + 3 n}{m}} {\binom{3 k + 2 n - 1}{n}}}{3 k + n}$$