$$U_{961}(x, y) = \frac{- 2 x y - \sqrt{4 x^{2} + 4 x y + 1} - 1}{2 y^{2} - 2}$$

$$T_{961}(n, m, k) = \begin{cases}0&\text{if n+m odd} ,\ \\1&\text{if n=0 , m=0} ,\ \\\frac{2 \left(-1\right)^{\frac{m}{2} + \frac{n}{2} + 1} k {\binom{m + n}{m}} {\binom{- k + n - 1}{\frac{m}{2} + \frac{n}{2} - 1}}}{m + n}&\text{if n+m even} \end{cases} $$