| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 18 |
| 5 | 90 |
| 6 | 540 |
| 7 | 3060 |
| 8 | 20700 |
| 9 | 145980 |
| 10 | 1459800 |
| 11 | 13854600 |
| 12 | 140059800 |
| 13 | 1514748600 |
| 14 | 15869034000 |
| 15 | 285268878000 |
| 16 | 4109761962000 |
| 17 | 59488383690000 |
| 18 | 935767530036000 |
| 19 | 13364309726748000 |
| 20 | 240338216104020000 |
| 21 | 4540941256642020000 |
| 22 | 79739974380153240000 |
[1,1,2,6,18,90,540,3060,20700,145980,1459800, 13854600,140059800,1514748600,15869034000, 285268878000,4109761962000,59488383690000, 935767530036000,13364309726748000, 240338216104020000,4540941256642020000, 79739974380153240000] |