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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 24 |
| 4 | 110 |
| 5 | 544 |
| 6 | 2828 |
| 7 | 15232 |
| 8 | 84246 |
| 9 | 475648 |
| 10 | 2730068 |
| 11 | 15882240 |
| 12 | 93438540 |
| 13 | 554967040 |
| 14 | 3323125528 |
| 15 | 20039827456 |
| 16 | 121597985254 |
| 17 | 741871845376 |
| 18 | 4548193111428 |
| 19 | 28004975116288 |
| 20 | 173113004348580 |
| 21 | 1073893324357632 |
[1,2,6,24,110,544,2828,15232,84246,475648,2730068, 15882240,93438540,554967040,3323125528, 20039827456,121597985254,741871845376, 4548193111428,28004975116288,173113004348580, 1073893324357632] |