$$U_{721}(x, y) = - \frac{\sqrt{4 y + 1}}{x \left(4 y + 1\right)^{\frac{3}{2}} - 1}$$

$$TLeftA_{271825}(n, k) = \begin{cases}1&\text{if n = 0},\\\frac {k {(-1)}^{n-1}} {n} {\binom{3n-k-1}{n-1}} &\text{if n > 0},\\\end{cases} $$$$Tsqrt(n, k) = \begin{cases}\binom{m}{n} 4^{n}&\text{if k even, where m =} \frac{k} {2},\\\frac {{(-1)}^{n-m} \binom{n}{m} \binom{2n}{n}} {\binom{2n}{2m}} &\text{if k odd, n > m, where m =} \frac{k+1} {2},\\\frac {\binom{2m}{2n} \binom{2n}{n}} {\binom{m}{n}}&\text{if k odd, n} \le \text{m, where m =} \frac{k+1} {2},\\\end{cases} $$$$T_{721}(n, m, k) = \frac{k \operatorname{TLeftA_{271825}}{\left(n,k + 3 n \right)} \operatorname{Tsqrt}{\left(m,k + 3 n \right)}}{k + 3 n}$$