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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 1 |
| 3 | 3 |
| 4 | 6 |
| 5 | 10 |
| 6 | 15 |
| 7 | 21 |
| 8 | 28 |
| 9 | 36 |
| 10 | 45 |
| 11 | 55 |
| 12 | 66 |
| 13 | 78 |
| 14 | 91 |
| 15 | 171 |
| 16 | 276 |
| 17 | 378 |
| 18 | 1770 |
| 19 | 2775 |
| 20 | 5778 |
| 21 | 8778 |
[0,1,3,6,10,15,21,28,36,45,55,66,78,91,171,276, 378,1770,2775,5778,8778] |