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| n | a(n) |
|---|---|
| 0 | 2 |
| 1 | 6 |
| 2 | 15 |
| 3 | 63 |
| 4 | 225 |
| 5 | 693 |
| 6 | 1289925 |
| 7 | 4455 |
| 8 | 34459425 |
| 9 | 808782975 |
| 10 | 5685805125 |
| 11 | 4106936925 |
| 12 | 18767808934875 |
| 13 | 72977109975 |
| 14 | 491292329653125 |
| 15 | 305714614450620375 |
| 16 | 1578522255175490625 |
| 17 | 33864491287501875 |
| 18 | 6076788748684677645496875 |
| 19 | 34996278233163121875 |
| 20 | 55478375013295336399171875 |
[2,6,15,63,225,693,1289925,4455,34459425, 808782975,5685805125,4106936925,18767808934875, 72977109975,491292329653125,305714614450620375, 1578522255175490625,33864491287501875, 6076788748684677645496875,34996278233163121875, 55478375013295336399171875] |