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| n | a(n) |
|---|---|
| -1 | 0 |
| 0 | 6 |
| 1 | 28 |
| 2 | 108 |
| 3 | 396 |
| 4 | 1430 |
| 5 | 5148 |
| 6 | 18564 |
| 7 | 67184 |
| 8 | 244188 |
| 9 | 891480 |
| 10 | 3268760 |
| 11 | 12034980 |
| 12 | 44482230 |
| 13 | 165002460 |
| 14 | 614106900 |
| 15 | 2292665760 |
| 16 | 8583849780 |
| 17 | 32223863880 |
| 18 | 121267584360 |
| 19 | 457412818200 |
| 20 | 1729020452796 |
| 21 | 6548744132568 |
| 22 | 24849948274088 |
| 23 | 94460672942496 |
| 24 | 359656297841400 |
[0,6,28,108,396,1430,5148,18564,67184,244188, 891480,3268760,12034980,44482230,165002460, 614106900,2292665760,8583849780,32223863880, 121267584360,457412818200,1729020452796, 6548744132568,24849948274088,94460672942496, 359656297841400] |