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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 20 |
| 3 | 116 |
| 4 | 589 |
| 5 | 3001 |
| 6 | 15644 |
| 7 | 81179 |
| 8 | 420243 |
| 9 | 2177937 |
| 10 | 11287977 |
| 11 | 58494710 |
| 12 | 303132340 |
| 13 | 1570918959 |
| 14 | 8140881877 |
| 15 | 42188034539 |
| 16 | 218628945511 |
| 17 | 1132989453013 |
| 18 | 5871432030067 |
| 19 | 30427217632524 |
| 20 | 157681390442587 |
| 21 | 817144077007238 |
[4,20,116,589,3001,15644,81179,420243,2177937, 11287977,58494710,303132340,1570918959,8140881877, 42188034539,218628945511,1132989453013, 5871432030067,30427217632524,157681390442587, 817144077007238] |