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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
| 4 | 4 |
| 5 | 20 |
| 6 | 90 |
| 7 | 630 |
| 8 | 3360 |
| 9 | 30240 |
| 10 | 226800 |
| 11 | 2494800 |
| 12 | 23950080 |
| 13 | 311351040 |
| 14 | 3632428800 |
| 15 | 54486432000 |
| 16 | 747242496000 |
| 17 | 12703122432000 |
| 18 | 200074178304000 |
| 19 | 3801409387776000 |
| 20 | 67580611338240000 |
| 21 | 1419192838103040000 |
| 22 | 28100018194440192000 |
| 23 | 646300418472124416000 |
[1,1,1,4,20,90,630,3360,30240,226800,2494800, 23950080,311351040,3632428800,54486432000, 747242496000,12703122432000,200074178304000, 3801409387776000,67580611338240000, 1419192838103040000,28100018194440192000, 646300418472124416000] |