$$U_{266}(x, y) = \frac{1 - \sqrt{- 4 x - 4 y^{2} + 1}}{2 x + 2 y^{2}}$$

$$T_{266}(n, m, k) = \frac{k \left(\left(-1\right)^{m} + 1\right) {\binom{\frac{m}{2} + n}{n}} {\binom{k + m + 2 n - 1}{\frac{m}{2} + n}}}{2 k + m + 2 n}$$