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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 36 |
| 3 | 560 |
| 4 | 12650 |
| 5 | 376992 |
| 6 | 13983816 |
| 7 | 621216192 |
| 8 | 32164253550 |
| 9 | 1902231808400 |
| 10 | 126524771308936 |
| 11 | 9349109217266832 |
| 12 | 759825846558448488 |
| 13 | 67362990324409227840 |
| 14 | 6469590128756692755600 |
| 15 | 669128737063722082963200 |
| 16 | 74148585178960981066746390 |
| 17 | 8764445804610791475090220464 |
| 18 | 1100718583140907766992803808200 |
[1,4,36,560,12650,376992,13983816,621216192, 32164253550,1902231808400,126524771308936, 9349109217266832,759825846558448488, 67362990324409227840,6469590128756692755600, 669128737063722082963200, 74148585178960981066746390, 8764445804610791475090220464, 1100718583140907766992803808200] |