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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 4 |
| 2 | 20 |
| 3 | 68 |
| 4 | 196 |
| 5 | 512 |
| 6 | 1256 |
| 7 | 2936 |
| 8 | 6628 |
| 9 | 14528 |
| 10 | 31140 |
| 11 | 65414 |
| 12 | 135276 |
| 13 | 275656 |
| 14 | 555216 |
| 15 | 1105726 |
| 16 | 2182380 |
| 17 | 4268906 |
| 18 | 8290740 |
| 19 | 15984420 |
| 20 | 30638312 |
| 21 | 58369924 |
| 22 | 110665328 |
| 23 | 208734268 |
| 24 | 392103508 |
| 25 | 733311754 |
| 26 | 1366650536 |
| 27 | 2537201920 |
[0,4,20,68,196,512,1256,2936,6628,14528,31140, 65414,135276,275656,555216,1105726,2182380, 4268906,8290740,15984420,30638312,58369924, 110665328,208734268,392103508,733311754, 1366650536,2537201920] |