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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 6 |
| 3 | 40 |
| 4 | 268 |
| 5 | 1801 |
| 6 | 12120 |
| 7 | 81628 |
| 8 | 550040 |
| 9 | 3707635 |
| 10 | 24997966 |
| 11 | 168573824 |
| 12 | 1136933488 |
| 13 | 7668785996 |
| 14 | 51731557296 |
| 15 | 348991600660 |
| 16 | 2354505179952 |
| 17 | 15885669341751 |
| 18 | 107183855819490 |
| 19 | 723217053276952 |
| 20 | 4880016412621148 |
| 21 | 32929530655094281 |
[0,1,6,40,268,1801,12120,81628,550040,3707635, 24997966,168573824,1136933488,7668785996, 51731557296,348991600660,2354505179952, 15885669341751,107183855819490,723217053276952, 4880016412621148,32929530655094281] |