$$U_{27}(x, y) = \frac{- 2 x - 2 y - \sqrt{- 4 x - 4 y + 1} + 1}{2 \left(x + y\right)^{2}}$$

$$T_{27}(n, m, k) = \frac{k {\binom{m + n}{n}} {\binom{2 k + 2 m + 2 n}{m + n}}}{k + m + n}$$