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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 2 |
| 3 | 3 |
| 4 | 44 |
| 5 | 90 |
| 6 | 2394 |
| 7 | 6720 |
| 8 | 202544 |
| 9 | 1041768 |
| 10 | 27369000 |
| 11 | 170418600 |
| 12 | 5835999432 |
| 13 | 41711464080 |
| 14 | 1489935696144 |
| 15 | 14980499777880 |
| 16 | 519279726915840 |
| 17 | 5837621201012160 |
| 18 | 232228922844775104 |
| 19 | 2946339663605953920 |
| 20 | 122979308145781345920 |
| 21 | 1869847203939341074560 |
[1,0,2,3,44,90,2394,6720,202544,1041768,27369000, 170418600,5835999432,41711464080,1489935696144, 14980499777880,519279726915840,5837621201012160, 232228922844775104,2946339663605953920, 122979308145781345920,1869847203939341074560] |