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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 3 |
| 3 | 1 |
| 4 | 18 |
| 5 | 15 |
| 6 | 138 |
| 7 | 189 |
| 8 | 1218 |
| 9 | 2280 |
| 10 | 11826 |
| 11 | 27225 |
| 12 | 123013 |
| 13 | 325611 |
| 14 | 1346631 |
| 15 | 3919188 |
| 16 | 15318342 |
| 17 | 47563620 |
| 18 | 179405250 |
| 19 | 582336054 |
| 20 | 2148831144 |
| 21 | 7191954822 |
| 22 | 26193070008 |
| 23 | 89559039141 |
| 24 | 323765075223 |
| 25 | 1123859351610 |
| 26 | 4047466156545 |
[1,0,3,1,18,15,138,189,1218,2280,11826,27225, 123013,325611,1346631,3919188,15318342,47563620, 179405250,582336054,2148831144,7191954822, 26193070008,89559039141,323765075223, 1123859351610,4047466156545] |