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| n | a(n) |
|---|---|
| 1 | 2 |
| 2 | 16 |
| 3 | 72 |
| 4 | 420 |
| 5 | 2176 |
| 6 | 10660 |
| 7 | 60984 |
| 8 | 340816 |
| 9 | 1846240 |
| 10 | 11076228 |
| 11 | 66303360 |
| 12 | 390426916 |
| 13 | 2456007216 |
| 14 | 15586002640 |
| 15 | 98058552648 |
| 16 | 645057461220 |
| 17 | 4299810563584 |
| 18 | 28563593727716 |
| 19 | 195777751059000 |
| 20 | 1361449965542992 |
| 21 | 9468978890210480 |
[2,16,72,420,2176,10660,60984,340816,1846240, 11076228,66303360,390426916,2456007216, 15586002640,98058552648,645057461220, 4299810563584,28563593727716,195777751059000, 1361449965542992,9468978890210480] |