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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 73 |
| 5 | 334 |
| 6 | 2592 |
| 7 | 18919 |
| 8 | 164476 |
| 9 | 1558521 |
| 10 | 1 |
| 11 | 187044031 |
| 12 | 2326111614 |
| 13 | 31214008090 |
[0,1,4,18,73,334,2592,18919,164476,1558521,1, 187044031,2326111614,31214008090] |