$$U_{428}(x, y) = \frac{- 2 y - \sqrt{1 - 4 y} - \sqrt{2} \sqrt{8 x y^{4} + 2 y^{2} - 4 y + \sqrt{1 - 4 y} \left(2 y - 1\right) + 1} + 1}{4 y^{2}}$$

$$T_{428}(n, m, k) = \begin{cases}0&\text{if n<k} ,\ \\- \frac{k {\binom{k - n - 1}{n - 1}}}{n}&\text{if m=0} ,\ \\- \frac{2 k {\binom{k - n - 1}{n}} {\binom{2 k + 2 m - 4 n - 1}{m - 1}}}{m} \end{cases} $$