The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
| 4 | 4 |
| 5 | 40 |
| 6 | 190 |
| 7 | 1330 |
| 8 | 8680 |
| 9 | 52920 |
| 10 | 340200 |
| 11 | 6237000 |
| 12 | 76211520 |
| 13 | 1098857760 |
| 14 | 11677585920 |
| 15 | 109679169600 |
| 16 | 1497396700800 |
| 17 | 41977644508800 |
| 18 | 783593969558400 |
| 19 | 15973899557616000 |
| 20 | 263524120417958400 |
| 21 | 3733362595368806400 |
| 22 | 64262934423790502400 |
[1,1,1,1,4,40,190,1330,8680,52920,340200,6237000, 76211520,1098857760,11677585920,109679169600, 1497396700800,41977644508800,783593969558400, 15973899557616000,263524120417958400, 3733362595368806400,64262934423790502400] |