$$U_{1266}(x, y) = \frac{1 - \sqrt{- \frac{4 x}{\left(1 - y\right)^{3}} + 1}}{2 x}$$

$$T_{1266}(n, m, k) = \frac{k {\binom{k + 2 n - 1}{n}} {\binom{3 k + m + 3 n - 1}{m}}}{k + n}$$