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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 28 |
| 3 | 408 |
| 4 | 1502 |
| 5 | 7821 |
| 6 | 31911 |
| 7 | 145162 |
| 8 | 616196 |
| 9 | 2706385 |
| 10 | 11640499 |
| 11 | 50598522 |
| 12 | 218517332 |
| 13 | 946752849 |
| 14 | 4093542243 |
| 15 | 17716803778 |
| 16 | 76627964684 |
| 17 | 331523693857 |
| 18 | 1434000301795 |
| 19 | 6203258085066 |
| 20 | 26832402306020 |
| 21 | 116067057052689 |
[0,28,408,1502,7821,31911,145162,616196,2706385, 11640499,50598522,218517332,946752849,4093542243, 17716803778,76627964684,331523693857, 1434000301795,6203258085066,26832402306020, 116067057052689] |