| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 30 |
| 4 | 340 |
| 5 | 5235 |
| 6 | 102756 |
| 7 | 2464898 |
| 8 | 70120020 |
| 9 | 2313120225 |
| 10 | 86962820000 |
| 11 | 3674969314090 |
| 12 | 172615622432040 |
| 13 | 8928295918586815 |
| 14 | 504561763088722500 |
| 15 | 30946605756915149850 |
| 16 | 2048137516834986743700 |
| 17 | 145535818715694311408181 |
| 18 | 11054204297079333714850260 |
[1,1,4,30,340,5235,102756,2464898,70120020, 2313120225,86962820000,3674969314090, 172615622432040,8928295918586815, 504561763088722500,30946605756915149850, 2048137516834986743700,145535818715694311408181, 11054204297079333714850260] |