$$U_{915}(x, y) = \frac{x}{2} - \frac{\sqrt{x^{2} + 2 x + y \left(8 x + 8\right) + 1}}{2} + \frac{1}{2}$$

$$T_{915}(n, m, k) = \frac{\left(-1\right)^{k + m + n} 2^{k + m} k {\binom{k + 2 m - 1}{m}} {\binom{m + n - 1}{n}}}{k + m}$$