$$U_{901}(x, y) = \frac{1}{\left(1 - y\right)^{3} \left(- x \left(1 - y\right) + 1\right)^{2}}$$

$$T_{901}(n, m, k) = {\binom{2 k + n - 1}{n}} {\binom{3 k + m - n - 1}{m}}$$