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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 28 |
| 4 | 280 |
| 5 | 3639 |
| 6 | 57996 |
| 7 | 1093795 |
| 8 | 23821104 |
| 9 | 588282772 |
| 10 | 16243898516 |
| 11 | 495894495629 |
| 12 | 16584179388232 |
| 13 | 602955889304341 |
| 14 | 23678788166350620 |
| 15 | 998882687260157956 |
| 16 | 45047554811998482016 |
| 17 | 2162775743390757357579 |
| 18 | 110136661581764181626660 |
| 19 | 5929361362606879245799055 |
| 20 | 336484778280758295928357240 |
[1,1,4,28,280,3639,57996,1093795,23821104, 588282772,16243898516,495894495629,16584179388232, 602955889304341,23678788166350620, 998882687260157956,45047554811998482016, 2162775743390757357579,110136661581764181626660, 5929361362606879245799055, 336484778280758295928357240] |