$$U_{650}(x, y) = \frac{- \sqrt{4 y + 1} - 1}{\frac{x \left(\sqrt{4 y + 1} + 1\right)^{2}}{2} - 2}$$

$$T_{650}(n, m, k) = \begin{cases}1&\text{if n=0, m=0} ,\ \\\frac{\left(-1\right)^{m - 1} k {\binom{- k + 2 m - 2 n - 1}{m - 1}}}{m}&\text{if n=0, m>0} ,\ \\- \frac{k \left(n - 1\right) {\binom{- k - 1}{n - 1}}}{n}&\text{if m=0, n>0} ,\ \\\frac{\left(-1\right)^{m + n - 2} k \left(k + 2 n\right) {\binom{- k - 1}{n - 1}} {\binom{- k + 2 m - 2 n - 1}{m - 1}}}{m n}&\text{if n>0, m>0} \end{cases} $$