The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 1 |
| 3 | 4 |
| 4 | 22 |
| 5 | 140 |
| 6 | 1020 |
| 7 | 8400 |
| 8 | 77280 |
| 9 | 786240 |
| 10 | 8769600 |
| 11 | 106444800 |
| 12 | 1397088000 |
| 13 | 19718899200 |
| 14 | 297859161600 |
| 15 | 4794806016000 |
| 16 | 81947593728000 |
| 17 | 1482030950400000 |
| 18 | 28277150533632000 |
| 19 | 567677135241216000 |
| 20 | 11961768206868480000 |
| 21 | 263969867887165440000 |
[0,0,1,4,22,140,1020,8400,77280,786240,8769600, 106444800,1397088000,19718899200,297859161600, 4794806016000,81947593728000,1482030950400000, 28277150533632000,567677135241216000, 11961768206868480000,263969867887165440000] |