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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 16 |
| 3 | 60 |
| 4 | 228 |
| 5 | 864 |
| 6 | 3276 |
| 7 | 12420 |
| 8 | 47088 |
| 9 | 178524 |
| 10 | 676836 |
| 11 | 2566080 |
| 12 | 9728748 |
| 13 | 36884484 |
| 14 | 139839696 |
| 15 | 530172540 |
| 16 | 2010036708 |
| 17 | 7620627744 |
| 18 | 28891993356 |
| 19 | 109537863300 |
| 20 | 415289569968 |
| 21 | 1574482299804 |
| 22 | 5969315609316 |
| 23 | 22631393727360 |
| 24 | 85802128010028 |
| 25 | 325300565212164 |
[1,4,16,60,228,864,3276,12420,47088,178524,676836, 2566080,9728748,36884484,139839696,530172540, 2010036708,7620627744,28891993356,109537863300, 415289569968,1574482299804,5969315609316, 22631393727360,85802128010028,325300565212164] |