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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 44 |
| 4 | 684 |
| 5 | 13636 |
| 6 | 328000 |
| 7 | 9198240 |
| 8 | 294075040 |
| 9 | 10549834368 |
| 10 | 419626384128 |
| 11 | 18330935118080 |
| 12 | 872618259925632 |
| 13 | 44970631837229184 |
| 14 | 2494887017741434368 |
| 15 | 148272655438005392896 |
| 16 | 9399158287979230003200 |
| 17 | 633107847492164526284800 |
| 18 | 45159576693655485274008576 |
[1,1,4,44,684,13636,328000,9198240,294075040, 10549834368,419626384128,18330935118080, 872618259925632,44970631837229184, 2494887017741434368,148272655438005392896, 9399158287979230003200,633107847492164526284800, 45159576693655485274008576] |