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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 7 |
| 2 | 42 |
| 3 | 252 |
| 4 | 1512 |
| 5 | 9072 |
| 6 | 54432 |
| 7 | 326592 |
| 8 | 1959552 |
| 9 | 11757312 |
| 10 | 70543872 |
| 11 | 423263232 |
| 12 | 2539579392 |
| 13 | 15237476352 |
| 14 | 91424858112 |
| 15 | 548549148672 |
| 16 | 3291294892032 |
| 17 | 19747769352192 |
| 18 | 118486616113152 |
| 19 | 710919696678912 |
| 20 | 4265518180073472 |
[1,7,42,252,1512,9072,54432,326592,1959552, 11757312,70543872,423263232,2539579392, 15237476352,91424858112,548549148672, 3291294892032,19747769352192,118486616113152, 710919696678912,4265518180073472] |