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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 23 |
| 3 | 240 |
| 4 | 2355 |
| 5 | 22320 |
| 6 | 205942 |
| 7 | 1866592 |
| 8 | 16664147 |
| 9 | 147164464 |
| 10 | 1287061400 |
| 11 | 11174456704 |
| 12 | 96362903378 |
| 13 | 826641224992 |
| 14 | 7055950608008 |
| 15 | 59990171328640 |
| 16 | 508090131646771 |
| 17 | 4290071141315376 |
| 18 | 36113651041694280 |
| 19 | 303253863622924416 |
| 20 | 2540231448366573400 |
[1,2,23,240,2355,22320,205942,1866592,16664147, 147164464,1287061400,11174456704,96362903378, 826641224992,7055950608008,59990171328640, 508090131646771,4290071141315376, 36113651041694280,303253863622924416, 2540231448366573400] |