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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 18 |
| 3 | 124 |
| 4 | 930 |
| 5 | 7146 |
| 6 | 55804 |
| 7 | 441312 |
| 8 | 3521898 |
| 9 | 28307510 |
| 10 | 228820086 |
| 11 | 1858240956 |
| 12 | 15149110912 |
| 13 | 123905220292 |
| 14 | 1016261712240 |
| 15 | 8355494725376 |
| 16 | 68842600563918 |
| 17 | 568266625104498 |
| 18 | 4698576694639306 |
| 19 | 38906632384471820 |
| 20 | 322596353513983626 |
| 21 | 2678048134387075560 |
[1,2,18,124,930,7146,55804,441312,3521898, 28307510,228820086,1858240956,15149110912, 123905220292,1016261712240,8355494725376, 68842600563918,568266625104498,4698576694639306, 38906632384471820,322596353513983626, 2678048134387075560] |