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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 6 |
| 2 | 36 |
| 3 | 224 |
| 4 | 1440 |
| 5 | 9504 |
| 6 | 64064 |
| 7 | 439296 |
| 8 | 3055104 |
| 9 | 21498880 |
| 10 | 152807424 |
| 11 | 1095450624 |
| 12 | 7911587840 |
| 13 | 57511157760 |
| 14 | 420459724800 |
| 15 | 3089600348160 |
| 16 | 22806128885760 |
| 17 | 169033661153280 |
| 18 | 1257467341701120 |
| 19 | 9385880636620800 |
| 20 | 70271680244613120 |
| 21 | 527595313582571520 |
[1,6,36,224,1440,9504,64064,439296,3055104, 21498880,152807424,1095450624,7911587840, 57511157760,420459724800,3089600348160, 22806128885760,169033661153280,1257467341701120, 9385880636620800,70271680244613120, 527595313582571520] |