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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 22 |
| 4 | 139 |
| 5 | 958 |
| 6 | 6979 |
| 7 | 52851 |
| 8 | 411884 |
| 9 | 3281684 |
| 10 | 26609931 |
| 11 | 218874331 |
| 12 | 1821767351 |
| 13 | 15315464340 |
| 14 | 129859965329 |
| 15 | 1109239893974 |
| 16 | 9536166375605 |
| 17 | 82449167265098 |
| 18 | 716449009997437 |
| 19 | 6253709697731562 |
| 20 | 54808237437608982 |
| 21 | 482103739329417219 |
[1,1,4,22,139,958,6979,52851,411884,3281684, 26609931,218874331,1821767351,15315464340, 129859965329,1109239893974,9536166375605, 82449167265098,716449009997437,6253709697731562, 54808237437608982,482103739329417219] |