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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 28 |
| 3 | 232 |
| 4 | 2108 |
| 5 | 20364 |
| 6 | 205696 |
| 7 | 2149956 |
| 8 | 23087260 |
| 9 | 253400200 |
| 10 | 2831688428 |
| 11 | 32121034928 |
| 12 | 368996930720 |
| 13 | 4284878088040 |
| 14 | 50221403053556 |
| 15 | 593400572917032 |
| 16 | 7061298334083484 |
| 17 | 84555438345880932 |
| 18 | 1018170456984477856 |
| 19 | 12321676227943830972 |
[1,4,28,232,2108,20364,205696,2149956,23087260, 253400200,2831688428,32121034928,368996930720, 4284878088040,50221403053556,593400572917032, 7061298334083484,84555438345880932, 1018170456984477856,12321676227943830972] |