The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 2 |
| 3 | 2 |
| 4 | 18 |
| 5 | 228 |
| 6 | 1068 |
| 7 | 3696 |
| 8 | 11046 |
| 9 | 30390 |
| 10 | 77976 |
| 11 | 190978 |
| 12 | 455166 |
| 13 | 1063452 |
| 14 | 2445450 |
| 15 | 5561658 |
| 16 | 12522478 |
| 17 | 27987780 |
| 18 | 62097646 |
| 19 | 136990768 |
| 20 | 300684322 |
[1,2,2,18,228,1068,3696,11046,30390,77976,190978, 455166,1063452,2445450,5561658,12522478,27987780, 62097646,136990768,300684322] |