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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 20 |
| 3 | 108 |
| 4 | 465 |
| 5 | 2265 |
| 6 | 10920 |
| 7 | 52752 |
| 8 | 253176 |
| 9 | 1221305 |
| 10 | 5885873 |
| 11 | 28368956 |
| 12 | 136712788 |
| 13 | 658952404 |
| 14 | 3175970497 |
| 15 | 15307435585 |
| 16 | 73778034832 |
| 17 | 355593913600 |
| 18 | 1713880597360 |
| 19 | 8260513454657 |
| 20 | 39813787812289 |
| 21 | 191893397639092 |
| 22 | 924882430123988 |
[4,20,108,465,2265,10920,52752,253176,1221305, 5885873,28368956,136712788,658952404,3175970497, 15307435585,73778034832,355593913600, 1713880597360,8260513454657,39813787812289, 191893397639092,924882430123988] |