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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 8 |
| 3 | 40 |
| 4 | 240 |
| 5 | 1528 |
| 6 | 8976 |
| 7 | 51904 |
| 8 | 302328 |
| 9 | 1757848 |
| 10 | 10199400 |
| 11 | 59182576 |
| 12 | 343432200 |
| 13 | 1992680992 |
| 14 | 11561675184 |
| 15 | 67081902808 |
| 16 | 389213984040 |
| 17 | 2258240299432 |
| 18 | 13102425739632 |
| 19 | 76020937882840 |
| 20 | 441077255457840 |
| 21 | 2559151966018144 |
[1,8,40,240,1528,8976,51904,302328,1757848, 10199400,59182576,343432200,1992680992, 11561675184,67081902808,389213984040, 2258240299432,13102425739632,76020937882840, 441077255457840,2559151966018144] |