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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 25 |
| 4 | 168 |
| 5 | 1218 |
| 6 | 9240 |
| 7 | 72501 |
| 8 | 583440 |
| 9 | 4789070 |
| 10 | 39940888 |
| 11 | 337490426 |
| 12 | 2883046320 |
| 13 | 24858176900 |
| 14 | 216046612080 |
| 15 | 1890737146125 |
| 16 | 16647522481440 |
| 17 | 147365806700310 |
| 18 | 1310740896927000 |
| 19 | 11708311265437710 |
| 20 | 104989351441019760 |
[1,4,25,168,1218,9240,72501,583440,4789070, 39940888,337490426,2883046320,24858176900, 216046612080,1890737146125,16647522481440, 147365806700310,1310740896927000, 11708311265437710,104989351441019760] |