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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 6 |
| 2 | 40 |
| 3 | 273 |
| 4 | 1870 |
| 5 | 12816 |
| 6 | 87841 |
| 7 | 602070 |
| 8 | 4126648 |
| 9 | 28284465 |
| 10 | 193864606 |
| 11 | 1328767776 |
| 12 | 9107509825 |
| 13 | 62423800998 |
| 14 | 427859097160 |
| 15 | 2932589879121 |
| 16 | 20100270056686 |
| 17 | 137769300517680 |
| 18 | 944284833567073 |
| 19 | 6472224534451830 |
[1,6,40,273,1870,12816,87841,602070,4126648, 28284465,193864606,1328767776,9107509825, 62423800998,427859097160,2932589879121, 20100270056686,137769300517680,944284833567073, 6472224534451830] |