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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 4 |
| 3 | 13 |
| 4 | 53 |
| 5 | 266 |
| 6 | 800 |
| 7 | 5601 |
| 8 | 44809 |
| 9 | 403282 |
| 10 | 2016412 |
| 11 | 22180533 |
| 12 | 88722135 |
| 13 | 1153387756 |
| 14 | 8073714294 |
| 15 | 40368571473 |
| 16 | 645897143569 |
| 17 | 10980251440674 |
| 18 | 32940754322028 |
| 19 | 625874332118533 |
| 20 | 12517486642370661 |
| 21 | 12517486642370682 |
[1,2,4,13,53,266,800,5601,44809,403282,2016412, 22180533,88722135,1153387756,8073714294, 40368571473,645897143569,10980251440674, 32940754322028,625874332118533,12517486642370661, 12517486642370682] |