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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 4 |
| 3 | 21 |
| 4 | 204 |
| 5 | 2500 |
| 6 | 38709 |
| 7 | 730420 |
| 8 | 16409180 |
| 9 | 430786429 |
| 10 | 13019414196 |
| 11 | 447437830704 |
| 12 | 17306961847705 |
| 13 | 746907935199264 |
| 14 | 35695643204860420 |
| 15 | 1876878693983656605 |
| 16 | 107956500727342113004 |
| 17 | 6758630146906528885412 |
| 18 | 458470139353155531447869 |
[1,0,4,21,204,2500,38709,730420,16409180, 430786429,13019414196,447437830704,17306961847705, 746907935199264,35695643204860420, 1876878693983656605,107956500727342113004, 6758630146906528885412,458470139353155531447869] |