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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 26 |
| 4 | 210 |
| 5 | 1897 |
| 6 | 18368 |
| 7 | 186636 |
| 8 | 1965414 |
| 9 | 21277685 |
| 10 | 235493544 |
| 11 | 2653779856 |
| 12 | 30357956720 |
| 13 | 351719984280 |
| 14 | 4119552129280 |
| 15 | 48708104589368 |
| 16 | 580682799531822 |
| 17 | 6973356315752445 |
| 18 | 84286657672243880 |
| 19 | 1024694111031383100 |
| 20 | 12522664914160322460 |
| 21 | 153762682439070435390 |
[1,1,4,26,210,1897,18368,186636,1965414,21277685, 235493544,2653779856,30357956720,351719984280, 4119552129280,48708104589368,580682799531822, 6973356315752445,84286657672243880, 1024694111031383100,12522664914160322460, 153762682439070435390] |