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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 2 |
| 3 | 9 |
| 4 | 48 |
| 5 | 325 |
| 6 | 2640 |
| 7 | 24997 |
| 8 | 270592 |
| 9 | 3295017 |
| 10 | 44582400 |
| 11 | 663532001 |
| 12 | 10773295104 |
| 13 | 189494874413 |
| 14 | 3589475821568 |
| 15 | 72849709631805 |
| 16 | 1577078610001920 |
| 17 | 36275031333172945 |
| 18 | 883457851718762496 |
| 19 | 22711455593084360761 |
| 20 | 614582936996534026240 |
[1,1,2,9,48,325,2640,24997,270592,3295017, 44582400,663532001,10773295104,189494874413, 3589475821568,72849709631805,1577078610001920, 36275031333172945,883457851718762496, 22711455593084360761,614582936996534026240] |