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| n | a(n) |
|---|---|
| 1 | 2 |
| 2 | 4 |
| 3 | 28 |
| 4 | 204 |
| 5 | 2308 |
| 6 | 29996 |
| 7 | 510508 |
| 8 | 9699480 |
| 9 | 223092840 |
| 10 | 6469690916 |
| 11 | 200560490128 |
| 12 | 7420738104576 |
| 13 | 304250263527208 |
| 14 | 13082761331159516 |
| 15 | 614889782588489072 |
| 16 | 32589158477180345040 |
| 17 | 1922760350154212639068 |
| 18 | 117288381359406747860400 |
| 19 | 7858321551080267055879088 |
[2,4,28,204,2308,29996,510508,9699480,223092840, 6469690916,200560490128,7420738104576, 304250263527208,13082761331159516, 614889782588489072,32589158477180345040, 1922760350154212639068,117288381359406747860400, 7858321551080267055879088] |