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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 12 |
| 4 | 50 |
| 5 | 182 |
| 6 | 836 |
| 7 | 3480 |
| 8 | 16808 |
| 9 | 78600 |
| 10 | 398832 |
| 11 | 1998976 |
| 12 | 10791704 |
| 13 | 57418904 |
| 14 | 322714800 |
| 15 | 1821518336 |
| 16 | 10673756016 |
| 17 | 62904395664 |
| 18 | 383965822240 |
| 19 | 2356753705600 |
| 20 | 14896682388192 |
| 21 | 95002532773632 |
| 22 | 620122408189824 |
[1,1,4,12,50,182,836,3480,16808,78600,398832, 1998976,10791704,57418904,322714800,1821518336, 10673756016,62904395664,383965822240, 2356753705600,14896682388192,95002532773632, 620122408189824] |