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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 104 |
| 5 | 665 |
| 6 | 5430 |
| 7 | 45647 |
| 8 | 457008 |
| 9 | 4903380 |
| 10 | 58782190 |
| 11 | 735362309 |
| 12 | 10577094600 |
| 13 | 153445432699 |
| 14 | 2462688382448 |
| 15 | 41517435884340 |
| 16 | 752130797186176 |
| 17 | 13961847566945787 |
| 18 | 284698729314200574 |
| 19 | 5855054339061479491 |
[1,1,4,18,104,665,5430,45647,457008,4903380, 58782190,735362309,10577094600,153445432699, 2462688382448,41517435884340,752130797186176, 13961847566945787,284698729314200574, 5855054339061479491] |