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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 18 |
| 3 | 84 |
| 4 | 403 |
| 5 | 1976 |
| 6 | 9860 |
| 7 | 49912 |
| 8 | 255701 |
| 9 | 1323292 |
| 10 | 6907830 |
| 11 | 36331500 |
| 12 | 192339687 |
| 13 | 1024140336 |
| 14 | 5481165832 |
| 15 | 29469454640 |
| 16 | 159094662121 |
| 17 | 862087135988 |
| 18 | 4687164401114 |
| 19 | 25562520325828 |
| 20 | 139803777476859 |
| 21 | 766578879858024 |
[1,4,18,84,403,1976,9860,49912,255701,1323292, 6907830,36331500,192339687,1024140336,5481165832, 29469454640,159094662121,862087135988, 4687164401114,25562520325828,139803777476859, 766578879858024] |