| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 112 |
| 4 | 660 |
| 5 | 4004 |
| 6 | 24752 |
| 7 | 155040 |
| 8 | 980628 |
| 9 | 6249100 |
| 10 | 40060020 |
| 11 | 258048960 |
| 12 | 1668903600 |
| 13 | 10829900592 |
| 14 | 70480305440 |
| 15 | 459823234112 |
| 16 | 3006465218196 |
| 17 | 19694758782300 |
| 18 | 129235131438140 |
| 19 | 849311959095600 |
| 20 | 5589126007740660 |
| 21 | 36825913869817380 |
| 22 | 242910890642347200 |
[1,4,20,112,660,4004,24752,155040,980628,6249100, 40060020,258048960,1668903600,10829900592, 70480305440,459823234112,3006465218196, 19694758782300,129235131438140,849311959095600, 5589126007740660,36825913869817380, 242910890642347200] |