$$U_{391}(x, y) = \frac{\left(y + 1\right)^{2}}{2} - \frac{\sqrt{4 x \left(y + 1\right)^{2} + \left(y + 1\right)^{4}}}{2}$$

$$T_{391}(n, m, k) = \begin{cases}0&\text{if n<k} ,\ \\- \frac{k {\binom{2 k - 2 n}{m}} {\binom{k - n - 1}{n - 1}}}{n} \end{cases} $$