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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 28 |
| 3 | 232 |
| 4 | 2092 |
| 5 | 19864 |
| 6 | 195376 |
| 7 | 1971932 |
| 8 | 20303084 |
| 9 | 212400232 |
| 10 | 2251379688 |
| 11 | 24129199208 |
| 12 | 261067326544 |
| 13 | 2848016992032 |
| 14 | 31295785633532 |
| 15 | 346126420439512 |
| 16 | 3850363854970476 |
| 17 | 43057199315715676 |
| 18 | 483795646775017312 |
| 19 | 5459770924922887392 |
[1,4,28,232,2092,19864,195376,1971932,20303084, 212400232,2251379688,24129199208,261067326544, 2848016992032,31295785633532,346126420439512, 3850363854970476,43057199315715676, 483795646775017312,5459770924922887392] |