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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 84 |
| 4 | 404 |
| 5 | 1748 |
| 6 | 8212 |
| 7 | 36180 |
| 8 | 167572 |
| 9 | 746452 |
| 10 | 3427604 |
| 11 | 15370836 |
| 12 | 70212500 |
| 13 | 316145876 |
| 14 | 1439545876 |
| 15 | 6497879892 |
| 16 | 29530613908 |
| 17 | 133496692180 |
| 18 | 605986514708 |
| 19 | 2741933589588 |
| 20 | 12437717824916 |
| 21 | 56308655258324 |
| 22 | 255312140456980 |
[1,4,20,84,404,1748,8212,36180,167572,746452, 3427604,15370836,70212500,316145876,1439545876, 6497879892,29530613908,133496692180,605986514708, 2741933589588,12437717824916,56308655258324, 255312140456980] |