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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 4 |
| 3 | 20 |
| 4 | 156 |
| 5 | 1540 |
| 6 | 18268 |
| 7 | 253380 |
| 8 | 4024220 |
| 9 | 72006788 |
| 10 | 1433150940 |
| 11 | 31402068292 |
| 12 | 751081569820 |
| 13 | 19471227247620 |
| 14 | 543821277818972 |
| 15 | 16278774958879940 |
| 16 | 519912803924769948 |
| 17 | 17646698226141369220 |
| 18 | 634307673944308523740 |
| 19 | 24070514898945667594308 |
[1,2,4,20,156,1540,18268,253380,4024220,72006788, 1433150940,31402068292,751081569820, 19471227247620,543821277818972,16278774958879940, 519912803924769948,17646698226141369220, 634307673944308523740,24070514898945667594308] |