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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 22 |
| 4 | 157 |
| 5 | 1368 |
| 6 | 13995 |
| 7 | 163779 |
| 8 | 2152349 |
| 9 | 31326760 |
| 10 | 499539900 |
| 11 | 8651755865 |
| 12 | 161591063250 |
| 13 | 3235298221401 |
| 14 | 69085973044125 |
| 15 | 1566556077233897 |
| 16 | 37577774143362824 |
| 17 | 950374452156347514 |
| 18 | 25266942445833523121 |
| 19 | 704302695217027889560 |
| 20 | 20534611808242819061155 |
| 21 | 624893326066824277558519 |
[1,1,4,22,157,1368,13995,163779,2152349,31326760, 499539900,8651755865,161591063250,3235298221401, 69085973044125,1566556077233897,37577774143362824, 950374452156347514,25266942445833523121, 704302695217027889560,20534611808242819061155, 624893326066824277558519] |