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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 1 |
| 3 | 4 |
| 4 | 13 |
| 5 | 76 |
| 6 | 421 |
| 7 | 3361 |
| 8 | 26209 |
| 9 | 267688 |
| 10 | 2689201 |
| 11 | 33579811 |
| 12 | 412800961 |
| 13 | 6103089994 |
| 14 | 88754687113 |
| 15 | 1517513934301 |
| 16 | 25487131948321 |
| 17 | 495009722435176 |
| 18 | 9430633148123809 |
| 19 | 205154208873930763 |
| 20 | 4371962638221712801 |
| 21 | 105330237499426955926 |
[1,1,1,4,13,76,421,3361,26209,267688,2689201, 33579811,412800961,6103089994,88754687113, 1517513934301,25487131948321,495009722435176, 9430633148123809,205154208873930763, 4371962638221712801,105330237499426955926] |