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 | 126 |
| 5 | 966 |
| 6 | 8656 |
| 7 | 88544 |
| 8 | 1016380 |
| 9 | 12920156 |
| 10 | 179996816 |
| 11 | 2725070096 |
| 12 | 44521522024 |
| 13 | 780344770440 |
| 14 | 14599772973696 |
| 15 | 290311643773376 |
| 16 | 6112190642062096 |
| 17 | 135798496839920144 |
| 18 | 3174483084427144000 |
| 19 | 77872118431269176896 |
| 20 | 1999809157085214044896 |
[1,1,4,20,126,966,8656,88544,1016380,12920156, 179996816,2725070096,44521522024,780344770440, 14599772973696,290311643773376,6112190642062096, 135798496839920144,3174483084427144000, 77872118431269176896,1999809157085214044896] |