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| n | a(n) |
|---|---|
| 1 | 6 |
| 2 | 15 |
| 3 | 120 |
| 4 | 645 |
| 5 | 3990 |
| 6 | 23295 |
| 7 | 140610 |
| 8 | 839790 |
| 9 | 5042610 |
| 10 | 30232965 |
| 11 | 181421850 |
| 12 | 1088390415 |
| 13 | 6530486970 |
| 14 | 39182081385 |
| 15 | 235093327980 |
| 16 | 1410554953080 |
| 17 | 8463334761210 |
| 18 | 50779978307010 |
| 19 | 304679900238330 |
[6,15,120,645,3990,23295,140610,839790,5042610, 30232965,181421850,1088390415,6530486970, 39182081385,235093327980,1410554953080, 8463334761210,50779978307010,304679900238330] |