The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 0 |
| 3 | 2 |
| 4 | 20 |
| 5 | 204 |
| 6 | 2604 |
| 7 | 39990 |
| 8 | 720600 |
| 9 | 14913080 |
| 10 | 348678440 |
| 11 | 9090909090 |
| 12 | 261535698060 |
| 13 | 8230246567620 |
| 14 | 281241170407092 |
| 15 | 10371206370520814 |
| 16 | 410525522232055664 |
| 17 | 17361641481138401520 |
| 18 | 781282469559318055056 |
[0,0,2,20,204,2604,39990,720600,14913080, 348678440,9090909090,261535698060,8230246567620, 281241170407092,10371206370520814, 410525522232055664,17361641481138401520, 781282469559318055056] |