The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 1 |
| 3 | 4 |
| 4 | 13 |
| 5 | 76 |
| 6 | 421 |
| 7 | 3424 |
| 8 | 26713 |
| 9 | 285136 |
| 10 | 2871241 |
| 11 | 38302144 |
| 12 | 473357413 |
| 13 | 7603381696 |
| 14 | 111506036461 |
| 15 | 2097552753664 |
| 16 | 35624155784113 |
| 17 | 768471264000256 |
| 18 | 14838126699784081 |
| 19 | 361133162545705984 |
[1,1,1,4,13,76,421,3424,26713,285136,2871241, 38302144,473357413,7603381696,111506036461, 2097552753664,35624155784113,768471264000256, 14838126699784081,361133162545705984] |