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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 1 |
| 3 | 4 |
| 4 | 13 |
| 5 | 56 |
| 6 | 301 |
| 7 | 1688 |
| 8 | 11705 |
| 9 | 84160 |
| 10 | 698521 |
| 11 | 6141312 |
| 12 | 59340997 |
| 13 | 613282944 |
| 14 | 6782462597 |
| 15 | 80158806016 |
| 16 | 1000434618609 |
| 17 | 13267800137728 |
| 18 | 184576848771889 |
| 19 | 2710082835353600 |
| 20 | 41577074746699261 |
| 21 | 669033814167273472 |
[1,1,1,4,13,56,301,1688,11705,84160,698521, 6141312,59340997,613282944,6782462597,80158806016, 1000434618609,13267800137728,184576848771889, 2710082835353600,41577074746699261, 669033814167273472] |