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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 1 |
| 3 | 3 |
| 4 | 7 |
| 5 | 18 |
| 6 | 31 |
| 7 | 57 |
| 8 | 180 |
| 9 | 320 |
| 10 | 1092 |
| 11 | 2478 |
| 12 | 10368 |
| 13 | 25704 |
| 14 | 100800 |
| 15 | 393120 |
| 16 | 1965600 |
| 17 | 10326624 |
| 18 | 39657600 |
| 19 | 131580288 |
| 20 | 511223790 |
| 21 | 1469266560 |
| 22 | 4911616440 |
| 23 | 22120488000 |
| 24 | 95841111600 |
| 25 | 460020244320 |
| 26 | 2163425531904 |
| 27 | 9805180170240 |
[1,1,3,7,18,31,57,180,320,1092,2478,10368,25704, 100800,393120,1965600,10326624,39657600,131580288, 511223790,1469266560,4911616440,22120488000, 95841111600,460020244320,2163425531904, 9805180170240] |