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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 15 |
| 4 | 84 |
| 5 | 420 |
| 6 | 2640 |
| 7 | 15015 |
| 8 | 100100 |
| 9 | 612612 |
| 10 | 4232592 |
| 11 | 27159132 |
| 12 | 192203088 |
| 13 | 1274816400 |
| 14 | 9178678080 |
| 15 | 62386327575 |
| 16 | 455053212900 |
| 17 | 3151664844900 |
| 18 | 23222793594000 |
| 19 | 163256238965820 |
| 20 | 1212760632317520 |
| 21 | 8629643838226320 |
| 22 | 64534727833692480 |
| 23 | 463843356304664700 |
| 24 | 3488102039411078544 |
[1,1,4,15,84,420,2640,15015,100100,612612,4232592, 27159132,192203088,1274816400,9178678080, 62386327575,455053212900,3151664844900, 23222793594000,163256238965820,1212760632317520, 8629643838226320,64534727833692480, 463843356304664700,3488102039411078544] |