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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 3 |
| 2 | 9 |
| 3 | 30 |
| 4 | 100 |
| 5 | 330 |
| 6 | 1089 |
| 7 | 3597 |
| 8 | 11881 |
| 9 | 39240 |
| 10 | 129600 |
| 11 | 428040 |
| 12 | 1413721 |
| 13 | 4669203 |
| 14 | 15421329 |
| 15 | 50933190 |
| 16 | 168220900 |
| 17 | 555595890 |
| 18 | 1835008569 |
| 19 | 6060621597 |
| 20 | 20016873361 |
| 21 | 66111241680 |
| 22 | 218350598400 |
| 23 | 721163036880 |
| 24 | 2381839709041 |
[1,3,9,30,100,330,1089,3597,11881,39240,129600, 428040,1413721,4669203,15421329,50933190, 168220900,555595890,1835008569,6060621597, 20016873361,66111241680,218350598400,721163036880, 2381839709041] |