$$U_{661}(x, y) = \frac{x}{1 - 4 y} + \frac{1}{\sqrt{1 - 4 y}}$$

$$T_{661}(n, m, k) = 4^{m} {\binom{k}{n}} {\binom{\frac{k}{2} + m + \frac{n}{2} - 1}{m}}$$