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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 108 |
| 4 | 604 |
| 5 | 3444 |
| 6 | 19876 |
| 7 | 115644 |
| 8 | 676748 |
| 9 | 3977316 |
| 10 | 23451700 |
| 11 | 138634764 |
| 12 | 821221692 |
| 13 | 4872734676 |
| 14 | 28952250564 |
| 15 | 172222729308 |
| 16 | 1025461169772 |
| 17 | 6110912705220 |
| 18 | 36441840179284 |
| 19 | 217450403367852 |
| 20 | 1298228971572764 |
| 21 | 7754336590794804 |
| 22 | 46335720234364900 |
[1,4,20,108,604,3444,19876,115644,676748,3977316, 23451700,138634764,821221692,4872734676, 28952250564,172222729308,1025461169772, 6110912705220,36441840179284,217450403367852, 1298228971572764,7754336590794804, 46335720234364900] |