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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 2 |
| 3 | 4 |
| 4 | 20 |
| 5 | 96 |
| 6 | 552 |
| 7 | 3536 |
| 8 | 25104 |
| 9 | 194816 |
| 10 | 1637408 |
| 11 | 14792768 |
| 12 | 142761280 |
| 13 | 1464117760 |
| 14 | 15886137984 |
| 15 | 181667507456 |
| 16 | 2182268117248 |
| 17 | 27456279388160 |
| 18 | 360872502280704 |
| 19 | 4943580063237120 |
| 20 | 70437638474568704 |
| 21 | 1041911242274562048 |
| 22 | 15972832382065977344 |
| 23 | 253388070573020401664 |
[1,0,2,4,20,96,552,3536,25104,194816,1637408, 14792768,142761280,1464117760,15886137984, 181667507456,2182268117248,27456279388160, 360872502280704,4943580063237120, 70437638474568704,1041911242274562048, 15972832382065977344,253388070573020401664] |