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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 16 |
| 4 | 104 |
| 5 | 508 |
| 6 | 5136 |
| 7 | 28224 |
| 8 | 333360 |
| 9 | 2793888 |
| 10 | 34010208 |
| 11 | 276522240 |
| 12 | 5903380896 |
| 13 | 50068045536 |
| 14 | 892740867024 |
| 15 | 13555604385504 |
| 16 | 260760485969664 |
| 17 | 3084227796562768 |
| 18 | 91112167715233008 |
| 19 | 1145087508241888160 |
[1,1,4,16,104,508,5136,28224,333360,2793888, 34010208,276522240,5903380896,50068045536, 892740867024,13555604385504,260760485969664, 3084227796562768,91112167715233008, 1145087508241888160] |