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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 18 |
| 3 | 72 |
| 4 | 324 |
| 5 | 1360 |
| 6 | 6280 |
| 7 | 27296 |
| 8 | 128784 |
| 9 | 573760 |
| 10 | 2752032 |
| 11 | 12485248 |
| 12 | 60652608 |
| 13 | 278988032 |
| 14 | 1368979584 |
| 15 | 6365514240 |
| 16 | 31488733440 |
| 17 | 147697177600 |
| 18 | 735486927360 |
| 19 | 3474529986560 |
| 20 | 17398140748800 |
| 21 | 82683153633280 |
| 22 | 415966710171648 |
[1,4,18,72,324,1360,6280,27296,128784,573760, 2752032,12485248,60652608,278988032,1368979584, 6365514240,31488733440,147697177600,735486927360, 3474529986560,17398140748800,82683153633280, 415966710171648] |