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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 1 |
| 3 | 2 |
| 4 | 6 |
| 5 | 20 |
| 6 | 70 |
| 7 | 246 |
| 8 | 854 |
| 9 | 2920 |
| 10 | 9846 |
| 11 | 32810 |
| 12 | 108262 |
| 13 | 354300 |
| 14 | 1151462 |
| 15 | 3720094 |
| 16 | 11957430 |
| 17 | 38263760 |
| 18 | 121965718 |
| 19 | 387420498 |
| 20 | 1226831558 |
| 21 | 3874204900 |
| 22 | 12203745414 |
| 23 | 38354628422 |
| 24 | 120294061846 |
| 25 | 376572715320 |
| 26 | 1176789735350 |
| 27 | 3671583974266 |
[0,1,2,6,20,70,246,854,2920,9846,32810,108262, 354300,1151462,3720094,11957430,38263760, 121965718,387420498,1226831558,3874204900, 12203745414,38354628422,120294061846,376572715320, 1176789735350,3671583974266] |