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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 112 |
| 4 | 676 |
| 5 | 4304 |
| 6 | 28496 |
| 7 | 194240 |
| 8 | 1353508 |
| 9 | 9593104 |
| 10 | 68906320 |
| 11 | 500281280 |
| 12 | 3664176400 |
| 13 | 27033720640 |
| 14 | 200683238720 |
| 15 | 1497639994112 |
| 16 | 11227634469668 |
| 17 | 84509490017680 |
| 18 | 638344820152784 |
| 19 | 4836914483890112 |
| 20 | 36753795855173776 |
| 21 | 279985580271435584 |
| 22 | 2137790149251471680 |
[1,4,20,112,676,4304,28496,194240,1353508,9593104, 68906320,500281280,3664176400,27033720640, 200683238720,1497639994112,11227634469668, 84509490017680,638344820152784,4836914483890112, 36753795855173776,279985580271435584, 2137790149251471680] |