The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 1 |
| 3 | 0 |
| 4 | 1 |
| 5 | 4 |
| 6 | 12 |
| 7 | 55 |
| 8 | 198 |
| 9 | 979 |
| 10 | 4700 |
| 11 | 22288 |
| 12 | 131612 |
| 13 | 754614 |
| 14 | 4833776 |
| 15 | 32539094 |
| 16 | 225628769 |
| 17 | 1675660796 |
| 18 | 12676652631 |
| 19 | 100996809180 |
| 20 | 830086817650 |
| 21 | 7065404491242 |
[1,0,1,0,1,4,12,55,198,979,4700,22288,131612, 754614,4833776,32539094,225628769,1675660796, 12676652631,100996809180,830086817650, 7065404491242] |