$$U_{221}(x, y) = \frac{- x^{2} + \sqrt{x^{4} + y \sqrt{1 - 4 x} \left(2 - 4 x\right) + y \left(- 4 x^{2} + 8 x - 2\right)}}{y \sqrt{1 - 4 x} + y \left(2 x - 1\right)}$$

$$T_{221}(n, m, k) = \frac{k {\binom{k + 2 m}{m}} {\binom{2 k + 4 m + 2 n}{n}}}{k + 2 m + n}$$