$$U_{1426}(x, y) = \frac{\left(\frac{x}{\left(1 - y\right)^{3}} + 1\right)^{2}}{\left(1 - y\right)^{2}}$$

$$T_{1426}(n, m, k) = {\binom{2 k}{n}} {\binom{2 k + m + 3 n - 1}{m}}$$