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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 40 |
| 4 | 210 |
| 5 | 2016 |
| 6 | 13860 |
| 7 | 164736 |
| 8 | 1351350 |
| 9 | 18670080 |
| 10 | 174594420 |
| 11 | 2708858880 |
| 12 | 28109701620 |
| 13 | 479259648000 |
| 14 | 5421156741000 |
| 15 | 100069414502400 |
| 16 | 1218404977539750 |
| 17 | 24087296714342400 |
| 18 | 312723944235202500 |
| 19 | 6566957735804928000 |
| 20 | 90252130306279441500 |
| 21 | 2000107698962300928000 |
[1,2,6,40,210,2016,13860,164736,1351350,18670080, 174594420,2708858880,28109701620,479259648000, 5421156741000,100069414502400,1218404977539750, 24087296714342400,312723944235202500, 6566957735804928000,90252130306279441500, 2000107698962300928000] |