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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 6 |
| 3 | 18 |
| 4 | 180 |
| 5 | 1500 |
| 6 | 15930 |
| 7 | 191646 |
| 8 | 2580648 |
| 9 | 38683224 |
| 10 | 636068430 |
| 11 | 11392350090 |
| 12 | 220658360076 |
| 13 | 4594593295188 |
| 14 | 102333126352002 |
| 15 | 2427278515815510 |
| 16 | 61079333377870800 |
| 17 | 1625065147997303856 |
| 18 | 45576552142354413078 |
| 19 | 1343802083242003570818 |
| 20 | 41552482139458105525620 |
[1,0,6,18,180,1500,15930,191646,2580648,38683224, 636068430,11392350090,220658360076,4594593295188, 102333126352002,2427278515815510, 61079333377870800,1625065147997303856, 45576552142354413078,1343802083242003570818, 41552482139458105525620] |