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| n | a(n) |
|---|---|
| 1 | 2 |
| 2 | 20 |
| 3 | 126 |
| 4 | 728 |
| 5 | 4374 |
| 6 | 25788 |
| 7 | 149794 |
| 8 | 866692 |
| 9 | 4989242 |
| 10 | 28600208 |
| 11 | 163479486 |
| 12 | 932166044 |
| 13 | 5304572502 |
| 14 | 30137247224 |
| 15 | 170986774826 |
| 16 | 968989758580 |
| 17 | 5485913976922 |
| 18 | 31032085382272 |
| 19 | 175409808771550 |
| 20 | 990873450379996 |
[2,20,126,728,4374,25788,149794,866692,4989242, 28600208,163479486,932166044,5304572502, 30137247224,170986774826,968989758580, 5485913976922,31032085382272,175409808771550, 990873450379996] |