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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 | 423263211 |
| 12 | 2539579140 |
| 13 | 15237474105 |
| 14 | 91424840220 |
| 15 | 548549014860 |
| 16 | 3291293930400 |
| 17 | 19747762629840 |
| 18 | 118486570063680 |
| 19 | 710919386089920 |
| 20 | 4265516110786560 |
[1,7,42,252,1512,9072,54432,326592,1959552, 11757312,70543872,423263211,2539579140, 15237474105,91424840220,548549014860, 3291293930400,19747762629840,118486570063680, 710919386089920,4265516110786560] |