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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 22 |
| 3 | 156 |
| 4 | 1368 |
| 5 | 14400 |
| 6 | 177840 |
| 7 | 2530080 |
| 8 | 40844160 |
| 9 | 738823680 |
| 10 | 14816390400 |
| 11 | 326439590400 |
| 12 | 7840777190400 |
| 13 | 203947385241600 |
| 14 | 5711834461132800 |
| 15 | 171375956623872000 |
| 16 | 5484386299392000000 |
| 17 | 186475536553033728000 |
[1,4,22,156,1368,14400,177840,2530080,40844160, 738823680,14816390400,326439590400,7840777190400, 203947385241600,5711834461132800, 171375956623872000,5484386299392000000, 186475536553033728000] |