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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 104 |
| 5 | 736 |
| 6 | 6232 |
| 7 | 61632 |
| 8 | 698144 |
| 9 | 8917120 |
| 10 | 126807520 |
| 11 | 1987075872 |
| 12 | 34018221728 |
| 13 | 631698903712 |
| 14 | 12645901972000 |
| 15 | 271482140140704 |
| 16 | 6221487421328672 |
| 17 | 151587364647728032 |
| 18 | 3912949321334320672 |
| 19 | 106670353381399285920 |
| 20 | 3062317963564624162592 |
| 21 | 92345208262957730327968 |
[1,1,4,18,104,736,6232,61632,698144,8917120, 126807520,1987075872,34018221728,631698903712, 12645901972000,271482140140704,6221487421328672, 151587364647728032,3912949321334320672, 106670353381399285920,3062317963564624162592, 92345208262957730327968] |