$$U_{281}(x, y) = \frac{1 - \sqrt{y \left(- 4 x^{2} - 4\right) + 1}}{2 y}$$

$$T_{281}(n, m, k) = \frac{k \left(\left(-1\right)^{n} + 1\right) {\binom{k + m}{\frac{n}{2}}} {\binom{k + 2 m - 1}{m}}}{2 k + 2 m}$$