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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 21 |
| 4 | 91 |
| 5 | 470 |
| 6 | 2763 |
| 7 | 17894 |
| 8 | 125113 |
| 9 | 932702 |
| 10 | 7347025 |
| 11 | 60761449 |
| 12 | 524852444 |
| 13 | 4716259252 |
| 14 | 43936770258 |
| 15 | 423178496553 |
| 16 | 4203717419747 |
| 17 | 42980494150963 |
| 18 | 451494062943969 |
| 19 | 4865135046178557 |
| 20 | 53701937703205383 |
| 21 | 606454041389250791 |
[1,2,6,21,91,470,2763,17894,125113,932702,7347025, 60761449,524852444,4716259252,43936770258, 423178496553,4203717419747,42980494150963, 451494062943969,4865135046178557, 53701937703205383,606454041389250791] |