The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 21 |
| 4 | 132 |
| 5 | 960 |
| 6 | 7944 |
| 7 | 74335 |
| 8 | 786932 |
| 9 | 9456258 |
| 10 | 129313528 |
| 11 | 2010416336 |
| 12 | 35343702312 |
| 13 | 696506363140 |
| 14 | 15229800217104 |
| 15 | 365825127164457 |
| 16 | 9567409932676756 |
| 17 | 270380073725933200 |
| 18 | 8204936841573636120 |
| 19 | 265952811211238746138 |
| 20 | 9166927690425432138872 |
| 21 | 334710118083817942361700 |
[1,1,4,21,132,960,7944,74335,786932,9456258, 129313528,2010416336,35343702312,696506363140, 15229800217104,365825127164457,9567409932676756, 270380073725933200,8204936841573636120, 265952811211238746138,9166927690425432138872, 334710118083817942361700] |