$$U_{907}(x, y) = \frac{\sqrt{- 4 x y^{2} - 16 x y - 16 x + 1} + 1}{2 y + 4}$$

$$T_{907}(n, m, k) = \begin{cases}1&\text{if n=0, m=0} ,\ \\-2&\text{if 2*n-k=0, m=0 } ,\ \\0&\text{if 2*n-k=0, m>0} ,\ \\- \frac{2^{- k - m + 2 n} k {\binom{- k + 2 n}{m}} {\binom{- k + 2 n}{n}}}{- k + 2 n} \end{cases} $$