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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 4 |
| 2 | 20 |
| 3 | 148 |
| 4 | 916 |
| 5 | 6036 |
| 6 | 38804 |
| 7 | 251796 |
| 8 | 1628052 |
| 9 | 10540948 |
| 10 | 68212628 |
| 11 | 441505684 |
| 12 | 2857424788 |
| 13 | 18493790100 |
| 14 | 119693957012 |
| 15 | 774676469652 |
| 16 | 5013809190804 |
| 17 | 32450060277652 |
| 18 | 210021188163476 |
| 19 | 1359285717096340 |
| 20 | 8797481879000980 |
[0,4,20,148,916,6036,38804,251796,1628052, 10540948,68212628,441505684,2857424788, 18493790100,119693957012,774676469652, 5013809190804,32450060277652,210021188163476, 1359285717096340,8797481879000980] |