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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 116 |
| 4 | 756 |
| 5 | 5428 |
| 6 | 42356 |
| 7 | 355636 |
| 8 | 3188340 |
| 9 | 30333492 |
| 10 | 304716148 |
| 11 | 3218555700 |
| 12 | 35618229364 |
| 13 | 411717043252 |
| 14 | 4957730174836 |
| 15 | 62045057731892 |
| 16 | 805323357485684 |
| 17 | 10820999695801908 |
| 18 | 150271018666120564 |
| 19 | 2153476417340487476 |
[1,4,20,116,756,5428,42356,355636,3188340, 30333492,304716148,3218555700,35618229364, 411717043252,4957730174836,62045057731892, 805323357485684,10820999695801908, 150271018666120564,2153476417340487476] |