| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 31 |
| 3 | 110 |
| 4 | 531 |
| 5 | 2534 |
| 6 | 11405 |
| 7 | 53175 |
| 8 | 246040 |
| 9 | 1135117 |
| 10 | 5253716 |
| 11 | 24286811 |
| 12 | 112274796 |
| 13 | 519155469 |
| 14 | 2400230989 |
| 15 | 11097385328 |
| 16 | 51309028043 |
| 17 | 237225520918 |
| 18 | 1096808926323 |
| 19 | 5071081279846 |
| 20 | 23446053720513 |
| 21 | 108402467083427 |
| 22 | 501197081280420 |
[4,31,110,531,2534,11405,53175,246040,1135117, 5253716,24286811,112274796,519155469,2400230989, 11097385328,51309028043,237225520918, 1096808926323,5071081279846,23446053720513, 108402467083427,501197081280420] |