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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 31 |
| 4 | 320 |
| 5 | 3969 |
| 6 | 56080 |
| 7 | 876204 |
| 8 | 14860614 |
| 9 | 270231265 |
| 10 | 5223002719 |
| 11 | 106613106181 |
| 12 | 2287120272173 |
| 13 | 51367948203527 |
| 14 | 1204141944566399 |
| 15 | 29385603693050274 |
| 16 | 744943334951904519 |
| 17 | 19580887642660810193 |
| 18 | 532781828387893449124 |
| 19 | 14984377196395037979472 |
[1,1,4,31,320,3969,56080,876204,14860614, 270231265,5223002719,106613106181,2287120272173, 51367948203527,1204141944566399,29385603693050274, 744943334951904519,19580887642660810193, 532781828387893449124,14984377196395037979472] |