| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 2 |
| 3 | 4 |
| 4 | 12 |
| 5 | 40 |
| 6 | 144 |
| 7 | 544 |
| 8 | 2128 |
| 9 | 8544 |
| 10 | 35008 |
| 11 | 145792 |
| 12 | 615296 |
| 13 | 2625792 |
| 14 | 11311616 |
| 15 | 49124352 |
| 16 | 214838528 |
| 17 | 945350144 |
| 18 | 4182412288 |
| 19 | 18593224704 |
| 20 | 83015133184 |
| 21 | 372090122240 |
| 22 | 1673660915712 |
| 23 | 7552262979584 |
| 24 | 34178799378432 |
| 25 | 155096251351040 |
| 26 | 705533929816064 |
[0,1,2,4,12,40,144,544,2128,8544,35008,145792, 615296,2625792,11311616,49124352,214838528, 945350144,4182412288,18593224704,83015133184, 372090122240,1673660915712,7552262979584, 34178799378432,155096251351040,705533929816064] |