$$U_{780}(x, y) = \frac{1}{\left(1 - y\right)^{6} \left(- x + \frac{1}{\left(1 - y\right)^{2}}\right)^{2}}$$

$$T_{780}(n, m, k) = {\binom{2 k + n - 1}{n}} {\binom{2 k + m - 2 n - 1}{m}}$$