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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 23 |
| 4 | 180 |
| 5 | 1777 |
| 6 | 21144 |
| 7 | 294239 |
| 8 | 4686680 |
| 9 | 84066001 |
| 10 | 1676633340 |
| 11 | 36801867479 |
| 12 | 881568186156 |
| 13 | 22883970972577 |
| 14 | 639869619046000 |
| 15 | 19173204600407423 |
| 16 | 612902677593991536 |
| 17 | 20819517833595304801 |
| 18 | 748889739331836981300 |
[0,1,4,23,180,1777,21144,294239,4686680,84066001, 1676633340,36801867479,881568186156, 22883970972577,639869619046000,19173204600407423, 612902677593991536,20819517833595304801, 748889739331836981300] |