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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 28 |
| 4 | 156 |
| 5 | 984 |
| 6 | 7112 |
| 7 | 57488 |
| 8 | 508688 |
| 9 | 4887840 |
| 10 | 50639200 |
| 11 | 561416640 |
| 12 | 6621963712 |
| 13 | 82719611264 |
| 14 | 1089925195904 |
| 15 | 15094182365440 |
| 16 | 219035852333312 |
| 17 | 3321573144437248 |
| 18 | 52511336993334784 |
| 19 | 863594855161338880 |
| 20 | 14746060593172585472 |
| 21 | 260971042697438779392 |
[1,2,6,28,156,984,7112,57488,508688,4887840, 50639200,561416640,6621963712,82719611264, 1089925195904,15094182365440,219035852333312, 3321573144437248,52511336993334784, 863594855161338880,14746060593172585472, 260971042697438779392] |