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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 4 |
| 3 | 55 |
| 4 | 752 |
| 5 | 8238 |
| 6 | 77960 |
| 7 | 663054 |
| 8 | 5215498 |
| 9 | 38678959 |
| 10 | 274025004 |
| 11 | 1872735358 |
| 12 | 12435556049 |
| 13 | 80677916282 |
| 14 | 513571982149 |
| 15 | 3218529210011 |
| 16 | 19909810689812 |
| 17 | 121825998303075 |
| 18 | 738596797827110 |
| 19 | 4442877434131554 |
| 20 | 26546050939783252 |
[0,4,55,752,8238,77960,663054,5215498,38678959, 274025004,1872735358,12435556049,80677916282, 513571982149,3218529210011,19909810689812, 121825998303075,738596797827110,4442877434131554, 26546050939783252] |