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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 28 |
| 3 | 252 |
| 4 | 2776 |
| 5 | 35940 |
| 6 | 533304 |
| 7 | 8908228 |
| 8 | 165247072 |
| 9 | 3368072196 |
| 10 | 74782987240 |
| 11 | 1796037420804 |
| 12 | 46379441090448 |
| 13 | 1281203788073092 |
| 14 | 37694510810334616 |
| 15 | 1176606639075726660 |
| 16 | 38833052393329645504 |
| 17 | 1351066066253778043908 |
| 18 | 49417629820950190273992 |
[1,4,28,252,2776,35940,533304,8908228,165247072, 3368072196,74782987240,1796037420804, 46379441090448,1281203788073092,37694510810334616, 1176606639075726660,38833052393329645504, 1351066066253778043908,49417629820950190273992] |