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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 4 |
| 3 | 4 |
| 4 | 36 |
| 5 | 68 |
| 6 | 356 |
| 7 | 900 |
| 8 | 3748 |
| 9 | 10948 |
| 10 | 40932 |
| 11 | 128516 |
| 12 | 455972 |
| 13 | 1484100 |
| 14 | 5131876 |
| 15 | 17004676 |
| 16 | 58059684 |
| 17 | 194097092 |
| 18 | 658574564 |
| 19 | 2211351300 |
| 20 | 7479947812 |
| 21 | 25170758212 |
| 22 | 85010340708 |
| 23 | 286376406404 |
| 24 | 966459132068 |
| 25 | 3257470383300 |
| 26 | 10989143439844 |
| 27 | 37048906506244 |
| 28 | 124962054024996 |
[1,0,4,4,36,68,356,900,3748,10948,40932,128516, 455972,1484100,5131876,17004676,58059684, 194097092,658574564,2211351300,7479947812, 25170758212,85010340708,286376406404,966459132068, 3257470383300,10989143439844,37048906506244, 124962054024996] |