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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 8 |
| 4 | 52 |
| 5 | 156 |
| 6 | 1536 |
| 7 | 6144 |
| 8 | 84096 |
| 9 | 420480 |
| 10 | 7453440 |
| 11 | 44720640 |
| 12 | 974972160 |
| 13 | 6824805120 |
| 14 | 176504832000 |
| 15 | 1412038656000 |
| 16 | 42224136192000 |
| 17 | 380017225728000 |
| 18 | 12893605517721600 |
| 19 | 128936055177216000 |
| 20 | 4892595136708608000 |
[1,1,4,8,52,156,1536,6144,84096,420480,7453440, 44720640,974972160,6824805120,176504832000, 1412038656000,42224136192000,380017225728000, 12893605517721600,128936055177216000, 4892595136708608000] |