The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 1 |
| 3 | 4 |
| 4 | 28 |
| 5 | 290 |
| 6 | 3996 |
| 7 | 68992 |
| 8 | 1434112 |
| 9 | 34895772 |
| 10 | 973450000 |
| 11 | 30636233936 |
| 12 | 1074020373504 |
| 13 | 41510792057176 |
| 14 | 1753764940408768 |
| 15 | 80412829785000000 |
| 16 | 3977094146761424896 |
| 17 | 211058327532167398928 |
| 18 | 11963018212810373415168 |
| 19 | 721321146876339731628352 |
[1,1,4,28,290,3996,68992,1434112,34895772, 973450000,30636233936,1074020373504, 41510792057176,1753764940408768,80412829785000000, 3977094146761424896,211058327532167398928, 11963018212810373415168,721321146876339731628352] |