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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 2 |
| 2 | 8 |
| 3 | 40 |
| 4 | 168 |
| 5 | 696 |
| 6 | 2776 |
| 7 | 10864 |
| 8 | 41800 |
| 9 | 158816 |
| 10 | 597176 |
| 11 | 2226512 |
| 12 | 8242344 |
| 13 | 30328160 |
| 14 | 111013784 |
| 15 | 404518640 |
| 16 | 1468154504 |
| 17 | 5309771264 |
| 18 | 19143323000 |
| 19 | 68823556368 |
| 20 | 246805713000 |
| 21 | 883028659744 |
| 22 | 3152718627672 |
| 23 | 11234773009200 |
[0,2,8,40,168,696,2776,10864,41800,158816,597176, 2226512,8242344,30328160,111013784,404518640, 1468154504,5309771264,19143323000,68823556368, 246805713000,883028659744,3152718627672, 11234773009200] |