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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 4 |
| 3 | 4 |
| 4 | 52 |
| 5 | 164 |
| 6 | 1364 |
| 7 | 7620 |
| 8 | 60148 |
| 9 | 449252 |
| 10 | 3831700 |
| 11 | 33811716 |
| 12 | 320082228 |
| 13 | 3178774564 |
| 14 | 33234163668 |
| 15 | 363535920196 |
| 16 | 4153091085172 |
| 17 | 49406896240996 |
| 18 | 610777358429204 |
| 19 | 7830140410294148 |
| 20 | 103914148870277556 |
| 21 | 1425254885630973604 |
| 22 | 20173671034640405588 |
[1,0,4,4,52,164,1364,7620,60148,449252,3831700, 33811716,320082228,3178774564,33234163668, 363535920196,4153091085172,49406896240996, 610777358429204,7830140410294148, 103914148870277556,1425254885630973604, 20173671034640405588] |