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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 4 |
| 3 | 4 |
| 4 | 20 |
| 5 | 36 |
| 6 | 116 |
| 7 | 260 |
| 8 | 724 |
| 9 | 1764 |
| 10 | 4660 |
| 11 | 11716 |
| 12 | 30356 |
| 13 | 77220 |
| 14 | 198644 |
| 15 | 507524 |
| 16 | 1302100 |
| 17 | 3332196 |
| 18 | 8540596 |
| 19 | 21869380 |
| 20 | 56031764 |
| 21 | 143509284 |
| 22 | 367636340 |
| 23 | 941673476 |
| 24 | 2412218836 |
| 25 | 6178912740 |
| 26 | 15827788084 |
| 27 | 40543439044 |
| 28 | 103854591380 |
[1,0,4,4,20,36,116,260,724,1764,4660,11716,30356, 77220,198644,507524,1302100,3332196,8540596, 21869380,56031764,143509284,367636340,941673476, 2412218836,6178912740,15827788084,40543439044, 103854591380] |