$$U_{251}(x, y) = \frac{- y^{2} - \sqrt{- 4 x + y^{4} - 2 y^{2} + 1} + 1}{2 x}$$

$$T_{251}(n, m, k) = \frac{k \left(\left(-1\right)^{m} + 1\right) {\binom{\frac{m}{2} + n}{n}} {\binom{k + \frac{m}{2} + 2 n - 1}{\frac{m}{2} + n}}}{2 k + 2 n}$$