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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 2 |
| 2 | 4 |
| 3 | 20 |
| 4 | 156 |
| 5 | 1664 |
| 6 | 22684 |
| 7 | 378572 |
| 8 | 7504640 |
| 9 | 172785512 |
| 10 | 4540756252 |
| 11 | 134330010172 |
| 12 | 4423176368332 |
| 13 | 160596613105384 |
| 14 | 6378859853838480 |
| 15 | 275308217428662672 |
| 16 | 12836003750434047344 |
| 17 | 643227594173121801096 |
[0,2,4,20,156,1664,22684,378572,7504640,172785512, 4540756252,134330010172,4423176368332, 160596613105384,6378859853838480, 275308217428662672,12836003750434047344, 643227594173121801096] |