Generating function
$$U_{921}(x, y) = \frac{\sqrt{- 4 x \left(y + 1\right) + 1} + 1}{2 y + 2}$$
Explicit formula
$$T_{921}(n, m, k) = \begin{cases}\left(-1\right)^{m} {\binom{k + m - 1}{m}}&\text{if n=0} ,\ \\- \frac{k {\binom{- k + n}{m}} {\binom{- k + 2 n - 1}{n - 1}}}{n} \end{cases} $$
| 1 | -1 | 1 | -1 | 1 | -1 | 1 |
| -1 | 0 | 0 | 0 | 0 | 0 | 0 |
| -1 | -1 | 0 | 0 | 0 | 0 | 0 |
| -2 | -4 | -2 | 0 | 0 | 0 | 0 |
| -5 | -15 | -15 | -5 | 0 | 0 | 0 |
| -14 | -56 | -84 | -56 | -14 | 0 | 0 |
| -42 | -21 | -42 | -42 | -21 | -42 | 0 |
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