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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 26 |
| 4 | 206 |
| 5 | 1592 |
| 6 | 12428 |
| 7 | 96632 |
| 8 | 752552 |
| 9 | 5856992 |
| 10 | 45595664 |
| 11 | 354918176 |
| 12 | 2762807264 |
| 13 | 21506309504 |
| 14 | 167411006144 |
| 15 | 1303169775488 |
| 16 | 10144214859392 |
| 17 | 78965192918528 |
| 18 | 614685592164608 |
| 19 | 4784872150020608 |
| 20 | 37246687385693696 |
[1,4,26,206,1592,12428,96632,752552,5856992, 45595664,354918176,2762807264,21506309504, 167411006144,1303169775488,10144214859392, 78965192918528,614685592164608,4784872150020608, 37246687385693696] |