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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 2 |
| 3 | 4 |
| 4 | 16 |
| 5 | 56 |
| 6 | 288 |
| 7 | 1576 |
| 8 | 9920 |
| 9 | 70912 |
| 10 | 541184 |
| 11 | 4668544 |
| 12 | 42274816 |
| 13 | 423673472 |
| 14 | 4449451776 |
| 15 | 50674728448 |
| 16 | 605255720960 |
| 17 | 7717401038848 |
| 18 | 103187353223168 |
| 19 | 1456461190426624 |
| 20 | 21528004660707328 |
| 21 | 333308651379949568 |
[0,1,2,4,16,56,288,1576,9920,70912,541184,4668544, 42274816,423673472,4449451776,50674728448, 605255720960,7717401038848,103187353223168, 1456461190426624,21528004660707328, 333308651379949568] |