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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 1 |
| 3 | 4 |
| 4 | 21 |
| 5 | 114 |
| 6 | 651 |
| 7 | 3844 |
| 8 | 23301 |
| 9 | 144169 |
| 10 | 906866 |
| 11 | 5782350 |
| 12 | 37289431 |
| 13 | 242793439 |
| 14 | 1593918916 |
| 15 | 10538988984 |
| 16 | 70121101825 |
| 17 | 469133993094 |
| 18 | 3154115695476 |
| 19 | 21299373321344 |
| 20 | 144402246424591 |
| 21 | 982506791975780 |
| 22 | 6706724412165956 |
| 23 | 45917245477282994 |
[1,0,1,4,21,114,651,3844,23301,144169,906866, 5782350,37289431,242793439,1593918916,10538988984, 70121101825,469133993094,3154115695476, 21299373321344,144402246424591,982506791975780, 6706724412165956,45917245477282994] |