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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 3 |
| 3 | 6 |
| 4 | 18 |
| 5 | 48 |
| 6 | 156 |
| 7 | 492 |
| 8 | 1740 |
| 9 | 6168 |
| 10 | 23568 |
| 11 | 91416 |
| 12 | 374232 |
| 13 | 1562640 |
| 14 | 6801888 |
| 15 | 30241488 |
| 16 | 139071696 |
| 17 | 653176992 |
| 18 | 3156467520 |
| 19 | 15566830368 |
| 20 | 78696180768 |
| 21 | 405599618496 |
| 22 | 2136915595392 |
| 23 | 11465706820800 |
| 24 | 62751681110208 |
| 25 | 349394351630208 |
| 26 | 1980938060495616 |
[1,1,3,6,18,48,156,492,1740,6168,23568,91416, 374232,1562640,6801888,30241488,139071696, 653176992,3156467520,15566830368,78696180768, 405599618496,2136915595392,11465706820800, 62751681110208,349394351630208,1980938060495616] |