The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 110 |
| 5 | 638 |
| 6 | 3828 |
| 7 | 23515 |
| 8 | 146972 |
| 9 | 930869 |
| 10 | 5958094 |
| 11 | 38462190 |
| 12 | 250054804 |
| 13 | 1635421543 |
| 14 | 10750864640 |
| 15 | 70987129653 |
| 16 | 470542935654 |
| 17 | 3129729034478 |
| 18 | 20880459397920 |
| 19 | 139689406647522 |
| 20 | 936832986074664 |
| 21 | 6297064070279195 |
[1,1,4,20,110,638,3828,23515,146972,930869, 5958094,38462190,250054804,1635421543,10750864640, 70987129653,470542935654,3129729034478, 20880459397920,139689406647522,936832986074664, 6297064070279195] |