| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 104 |
| 4 | 553 |
| 5 | 2984 |
| 6 | 16272 |
| 7 | 89440 |
| 8 | 494681 |
| 9 | 2749772 |
| 10 | 15348372 |
| 11 | 85967112 |
| 12 | 482927985 |
| 13 | 2719787856 |
| 14 | 15351385152 |
| 15 | 86816721792 |
| 16 | 491819758417 |
| 17 | 2790451952660 |
| 18 | 15854070902964 |
| 19 | 90187514559208 |
| 20 | 513619224125657 |
| 21 | 2928073006131704 |
| 22 | 16708228671139600 |
| 23 | 95423104768226144 |
| 24 | 545408567460801513 |
[1,4,20,104,553,2984,16272,89440,494681,2749772, 15348372,85967112,482927985,2719787856, 15351385152,86816721792,491819758417, 2790451952660,15854070902964,90187514559208, 513619224125657,2928073006131704, 16708228671139600,95423104768226144, 545408567460801513] |