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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 96 |
| 4 | 464 |
| 5 | 2240 |
| 6 | 10816 |
| 7 | 52224 |
| 8 | 252160 |
| 9 | 1217536 |
| 10 | 5878784 |
| 11 | 28385280 |
| 12 | 137056256 |
| 13 | 661766144 |
| 14 | 3195289600 |
| 15 | 15428222976 |
| 16 | 74494050304 |
| 17 | 359689093120 |
| 18 | 1736732573696 |
| 19 | 8385686667264 |
| 20 | 40489676963840 |
| 21 | 195501454524416 |
[1,4,20,96,464,2240,10816,52224,252160,1217536, 5878784,28385280,137056256,661766144,3195289600, 15428222976,74494050304,359689093120, 1736732573696,8385686667264,40489676963840, 195501454524416] |