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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 0 |
| 3 | 2 |
| 4 | 8 |
| 5 | 22 |
| 6 | 64 |
| 7 | 198 |
| 8 | 624 |
| 9 | 1994 |
| 10 | 6464 |
| 11 | 21210 |
| 12 | 70296 |
| 13 | 234990 |
| 14 | 791424 |
| 15 | 2682894 |
| 16 | 9147360 |
| 17 | 31347730 |
| 18 | 107919232 |
| 19 | 373055730 |
| 20 | 1294372008 |
| 21 | 4506163718 |
| 22 | 15735793088 |
| 23 | 55105084246 |
| 24 | 193471595344 |
| 25 | 680891484762 |
| 26 | 2401575077568 |
| 27 | 8487950090954 |
[1,1,0,2,8,22,64,198,624,1994,6464,21210,70296, 234990,791424,2682894,9147360,31347730,107919232, 373055730,1294372008,4506163718,15735793088, 55105084246,193471595344,680891484762, 2401575077568,8487950090954] |