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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 119 |
| 4 | 819 |
| 5 | 6397 |
| 6 | 55919 |
| 7 | 541144 |
| 8 | 5746596 |
| 9 | 66475311 |
| 10 | 832418065 |
| 11 | 11222752125 |
| 12 | 162133146877 |
| 13 | 2499401777680 |
| 14 | 40960858008040 |
| 15 | 711240364356155 |
| 16 | 13045720176453587 |
| 17 | 252079975222183461 |
| 18 | 5118581045978055067 |
| 19 | 108972887981432267708 |
| 20 | 2427417968714846394712 |
| 21 | 56467770394205361146187 |
[1,4,20,119,819,6397,55919,541144,5746596, 66475311,832418065,11222752125,162133146877, 2499401777680,40960858008040,711240364356155, 13045720176453587,252079975222183461, 5118581045978055067,108972887981432267708, 2427417968714846394712,56467770394205361146187] |