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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 1 |
| 3 | 0 |
| 4 | 4 |
| 5 | 16 |
| 6 | 86 |
| 7 | 542 |
| 8 | 3932 |
| 9 | 32330 |
| 10 | 297438 |
| 11 | 3028320 |
| 12 | 33814454 |
| 13 | 410954878 |
| 14 | 5400878692 |
| 15 | 76329470882 |
| 16 | 1154445436334 |
| 17 | 18606430004984 |
| 18 | 318369275913710 |
| 19 | 5764046146341198 |
| 20 | 110091446931897180 |
| 21 | 2212282487296335866 |
[1,1,0,4,16,86,542,3932,32330,297438,3028320, 33814454,410954878,5400878692,76329470882, 1154445436334,18606430004984,318369275913710, 5764046146341198,110091446931897180, 2212282487296335866] |