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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 28 |
| 3 | 219 |
| 4 | 1804 |
| 5 | 15314 |
| 6 | 132572 |
| 7 | 1163565 |
| 8 | 10316924 |
| 9 | 92195488 |
| 10 | 829016968 |
| 11 | 7492106505 |
| 12 | 67991427828 |
| 13 | 619193535380 |
| 14 | 5655829748520 |
| 15 | 51794730347745 |
| 16 | 475390078267356 |
| 17 | 4371917301657488 |
| 18 | 40276635724273936 |
[1,4,28,219,1804,15314,132572,1163565,10316924, 92195488,829016968,7492106505,67991427828, 619193535380,5655829748520,51794730347745, 475390078267356,4371917301657488, 40276635724273936] |