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| n | a(n) |
|---|---|
| 1 | 2 |
| 2 | 15 |
| 3 | 90 |
| 4 | 542 |
| 5 | 3255 |
| 6 | 19530 |
| 7 | 117182 |
| 8 | 703095 |
| 9 | 4218570 |
| 10 | 25311422 |
| 11 | 151868535 |
| 12 | 911211210 |
| 13 | 5467267262 |
| 14 | 32803603575 |
| 15 | 196821621450 |
| 16 | 1180929728702 |
| 17 | 7085578372215 |
| 18 | 42513470233290 |
| 19 | 255080821399742 |
| 20 | 1530484928398455 |
[2,15,90,542,3255,19530,117182,703095,4218570, 25311422,151868535,911211210,5467267262, 32803603575,196821621450,1180929728702, 7085578372215,42513470233290,255080821399742, 1530484928398455] |