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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 3 |
| 2 | 18 |
| 3 | 130 |
| 4 | 1020 |
| 5 | 8379 |
| 6 | 70840 |
| 7 | 610740 |
| 8 | 5340060 |
| 9 | 47187580 |
| 10 | 420412278 |
| 11 | 3770221338 |
| 12 | 33991902308 |
| 13 | 307826695050 |
| 14 | 2798052616800 |
| 15 | 25514463687720 |
| 16 | 233296537299228 |
| 17 | 2138295980859588 |
| 18 | 19639886707062280 |
| 19 | 180724535020583400 |
| 20 | 1665767679910654320 |
| 21 | 15376467276901980315 |
[1,3,18,130,1020,8379,70840,610740,5340060, 47187580,420412278,3770221338,33991902308, 307826695050,2798052616800,25514463687720, 233296537299228,2138295980859588, 19639886707062280,180724535020583400, 1665767679910654320,15376467276901980315] |